Content uploaded by Gennady M. Kamenev
Author content
All content in this area was uploaded by Gennady M. Kamenev on Nov 19, 2018
Content may be subject to copyright.
A preview of the PDF is not available
Two New Species of the Genus Silicula (Bivalvia: Siliculidae) from the Northwestern Pacific, with Notes on Silicula sandersi (Bernard, 1989) and Propeleda soyomaruae (Okutani, 1962)
Abstract and Figures
Two new species, Silicula beringiana and S. okutanii, are described from the northwestern
Pacific. These species have an elongate shell with extended and tapered posterior end, differing
from each other in form and proportions. Silicula beringiana was found in the Bering
Sea, Kamtchatky Strait, and near the southeastern coast of Kamchatka (Pacific Ocean) at
489–4,984 m depth. Silicula okutanii was found in the Pacific Ocean at a latitude of the northern
part of Japan at 5,219–5,352 m depth. Silicula sandersi (Bernard, 1989) and Propeleda
soyomaruae (Okutani, 1962) were re-examined. Based on comparison with the description,
pictures and photos of the type species of Silicula Jeffreys, 1879, Lamellileda Cotton, 1930,
PoroledaHutton, 1893, and Propeleda Iredale, 1924, the following combinations are suggested:
Lamellileda sandersi Bernard, 1989, and L. soyomaruae (Okutani, 1962).
Figures - uploaded by Gennady M. Kamenev
Author content
All figure content in this area was uploaded by Gennady M. Kamenev
Content may be subject to copyright.
... Known only from the type locality (Sunda Strait). (Kamenev, 2014). The valves of the new species possess mostly chevronshaped teeth on both its anterior and posterior hinge plates, with several lamellate teeth nearest the umbo on the anterior and posterior plates. ...
... The valves of the new species possess mostly chevronshaped teeth on both its anterior and posterior hinge plates, with several lamellate teeth nearest the umbo on the anterior and posterior plates. This condition closely resembles the hinge condition in Propeleda ensicula (Angas, 1877), the type species of Propeleda (Iredale, 1924) (see Iredale, 1924: 181, 186;Heinberg, 1989;Kamenev, 2014 (Prashad, 1932a: pl. 1 fig. 37) are comparable to (albeit narrower than) the new species. ...
... Externally, members of Silicula also resemble the new species in overall shape (see Kamenev, 2014) . 1) is also very similar to the new species but the posterior region is arched dorsally in ikebei, whereas it is straight in the new species. ...
The diversity of benthic protobranch bivalves collected during the South Java Deep-Sea (SJADES) Biodiversity Expedition 2018 was evaluated. Living and dead material was brought up by dredging and trawling from depths ranging from 100 to 2,000 m across 37 stations off the southeast and southwest coasts of Sumatra and Java respectively in the Sunda Strait and Indian Ocean. A total of 37 species in 17 genera belonging to six families (Nuculidae: Neilonellidae: Neilonella) were identified from some 961 dead valves and 171 intact, living individuals. Of these, 15 are regarded as new records for Indonesia. Amongst nine nuculid species found, the large Acila fultoni was surprisingly common and widely distributed in the area and is a new record for Indonesia. Two tiny nucinellids in the genera Nucinella and Huxleyia, represented only by dead shells, are also new Indonesian records. Not unexpectedly, the suborder Nuculanida comprised the most diverse group, with 11 nuculanids, seven malletiids, and five neilonellids. The most widely distributed, relatively large, deeper-water protobranchs were the nuculids Nucula donaciformis, Acila fultoni, Ennucula bengalensis, the nuculanid Sacella sibogai, and the malletiids Malletia arrouana, M. encrypta, and Carinineilo sp. They were all represented by living individuals. In contrast, at least 15 species were relatively rare, mostly represented only by dead shells, with many reported just from a single locality. These included the solemyids, nucinellids, as well as several nuculanids, malletiids, and neilonellids. A new species of Propeleda (Nuculanidae) is also described from 1,600 m in the Sunda Strait. Despite the restricted geographical coverage of this study, the diversity of protobranchs observed is comparable to the 35 species documented from the Siboga Expedition in the eastern half of the Indonesian archipelago a century ago.
... Recently, active studies of deep-sea bottom fauna have been resumed in the northwestern Pacific (Malyutina and Brandt, 2013;Brandt and Malyutina, 2015;Malyutina et al., 2018). Studies that were performed in deep-water basins of the Sea of Japan and the Sea of Okhotsk, as well as at the abyssal plain adjacent to the Kuril-Kamchatka Trench revealed a rich fauna of bivalves with many species new to science (Kamenev, 2013(Kamenev, , 2014(Kamenev, , 2015(Kamenev, , 2018aKrylova et al., 2015;Schwabe et al., 2015). In 2016, a KuramBio II expedition was undertaken to explore the bottom fauna of the hadal zone of the Kuril-Kamchatka Trench, as well as the abyssal zone of the Pacific slope of the Kuril Islands, which passes into the landward slope of the trench, and a part of the abyssal oceanic plain adjacent to the seaward slope. ...
... Family Siliculidae Allen and Sanders, 1973 Silicula okutanii Kamenev, 2014 ( Fig. 10E and F) Remarks: Earlier, this species was found only at abyssal depths of the oceanic plain and slope of the Kuril Islands on both sides of the Kuril-Kamchatka Trench (Kamenev, 2014(Kamenev, , 2015(Kamenev, , 2018c. The KuramBio II expedition collected this species in the Kuril-Kamchatka Trench at a depth of 6561 m. ...
... Thus, this finding is the first record of occurrence of Siliculidae species in the hadal zone. (Kamenev, 2014(Kamenev, , 2015(Kamenev, , 2018c. ...
The Kuril-Kamchatka Trench with a maximum depth of 9600 m is one of the deepest trenches of the World Ocean and has the second (to the Izu-Bonin Trench) largest area of hadal habitat. Investigation of the materials collected by the KuramBio II German-Russian expedition (RV Sonne, 2016) (8 stations), and three Russian expeditions (RV Vityaz, 1949, 1953, 1966) (16 stations) from the hadal zone of the Kuril-Kamchatka Trench (6000–9583 m depth) revealed a rich fauna of bivalves including 33 species (identified species and morphospecies) belonging to 15 families, of which 18 species (54.0%) were determined to the species level and 14 species (42.4%) were first recorded for the Kuril-Kamchatka Trench. Of the 33 species found in the Kuril-Kamchatka Trench, 15 species (45.5%) were recorded outside the trench at depths of less than 6000 m. Vesicomya sergeevi Filatova, 1971 and Parayoldiella ultraabyssalis (Filatova, 1971) formed the most abundant populations in the trench (72.9% of all specimens of bivalves collected by the KuramBio II expedition) and were predominant in samples from the trench bottom. In the hadal zone of the Kuril-Kamchatka Trench, 3 depth ranges (6000–6999, 7000–8999, 9000–9583 m) were distinguished that differ in the species composition of bivalves. The bivalve fauna of the upper hadal zone of the Kuril-Kamchatka Trench (depths 6000–6999 m) is characterized by the highest species richness (23 species) and is represented mainly (65.2%) by bathyal-hadal and abyssal-hadal species. This depth range is considered as a zone of transition between the abyssal and hadal zones. The fauna of this transitional zone is most closely connected with the fauna of the abyssal plain adjacent to the trench. At depths of more than 7000 m, the proportion of hadal endemics significantly increases (more than 66%). At the maximum depth of the trench (over 9500 m), only 5 species were recorded. Of the 18 identified species, only 6 species (33.3%) were found exclusively in the hadal zone of the trench. Photographs, taxonomic remarks and distribution are provided for all identified species that were found in the hadal zone of the Kuril-Kamchatka Trench. Taxonomic decisions herein: Bathyspinula vityazi (Filatova, 1964) is a distinct species and not a synonym of Bathyspinula calcarella (Dall, 1908); Ledella spinuliformis F. R. Bernard, 1989 is synonymized with Ledellina formabile Filatova and Schileyko, 1984; following Krylova (1993) Myonera demistrata Allen and Morgan, 1981 is considered as a synonym of Bathyneaera hadalis (Knudsen, 1970).
... Poroleda lanceolata has numerous oblique lamellate teeth in the posterior set and chevron-shaped teeth in the anterior set. Despite that the representatives of the genus Propeleda have a shell shape similar to that of N. extenuata, the type species of the genus L. ensicula has chevron-shaped posterior and anterior teeth with equal length dorsal and ventral arms, except several proximal teeth in the posterior set, which may have the shape of small lamellae (Allen and Sanders, 1996;Angas, 1877;Hedley, 1915;Heinberg, 1989;Kamenev, 2014;Nijssen-Meyer, 1972). Therefore, in contrast to Huber (2010), I think that N. extenuata should be referred to the genus Poroleda, but not to Propeleda. ...
... (Fig. 6A, B). Remarks: This species was previously found at the abyssal plain of the Pacific Ocean (depth 5219-5352 m) adjacent to the Kuril-Kamchatka Trench at a latitude of the northern part of Japan (Kamenev, 2014). However, Kamenev (2015) supposed that this species may have a wider range because juveniles of presumably this species were found to the north of Japan. ...
... Distribution: Pacific slope of the Kuril Islands and the abyssal plain of the Pacific Ocean adjacent to the Kuril-Kamchatka Trench at depths of 5013-5352 m (Kamenev, 2014(Kamenev, , 2015. ...
SokhoBio expedition Until recently hardly anything was known about the abyssal bivalve fauna of the Sea of Okhotsk, which communicates with the Pacific Ocean via many straits between the Kuril Islands with a maximum depth of 2318. m in Bussol Strait, and about its similarity with the deep-water fauna of the Pacific. Investigation of the materials collected by the SokhoBio Russian-German deep-sea expedition (RV "Akademik M.A. Lavrentyev", 2015), and two Russian expeditions (RV "Toporok", 1948; RV "Vityaz", 1949) from the bottom of the Kuril Basin of the Sea of Okhotsk (2850-3366. m depth) revealed a rich fauna of bivalves including 25 species belonging to 12 families, of which only 12 species (48.0%) were determined to the species level. Nine species (Poroleda extenuata, Katadesmia vincula, Dacrydium rostriferum, Bathyarca imitata, Catillopecten squamiformis, Channelaxinus excavata, Vesicomya pacifica, Cuspidaria cf. abyssopacifica, Myonera garretti) were first records for the Sea of Okhotsk. The richest families were Thyasiridae (7 species) and Cuspidariidae (6 species). The high diversity and richness of the bivalve fauna on the bottom of the Kuril Basin are probably caused by the favorable trophic conditions. The Kuril Basin is characterized by very high rates of sedimentation and high content of organic matter in the sediments. For estimation of connections between deep-water faunas of the Sea of Okhotsk and the Pacific Ocean, the SokhoBio expedition performed 1 station at the maximum depth of the Bussol Strait and 2 stations (3342-3432 and 4679-5013. m) on the Pacific slope of the Kuril Islands opposite to the Bussol Strait, where 24 species belonging to 13 families were found. Among the 25 species found in the deep part of the Kuril Basin more than half of species (17 species, 68%) occur in the Pacific Ocean. Most of them are widespread in the northern Pacific and are eurybathic bathyal-abyssal species. This probably allows them to penetrate into the Sea of Okhotsk through deep straits between the Kuril Islands. Photographs and taxonomic remarks are provided for all identified species (15) that were found in the Sea of Okhotsk and/or the Pacific Ocean. Taxonomic decisions herein: Nuculana aikawai Habe, 1958 and Nuculana sagamiensis Okutani, 1962 are synonymized with Nuculana leonina (Dall, 1896); Arca (Bathyarca) nucleator Dall, 1908 is synonymized with Bathyarca imitata (Smith, 1885); for Leda extenuata Dall, 1897, a new combination is suggested, Poroleda extenuata (Dall, 1897).
... Additional material comes from the collection of the Museo de Zoología de la Universidad de Concepció n (MZUC), Chile, and material sampled by the authors at the Beagle Channel and the South Shetland (61 23 0 45 00 S, 55 26 0 33 00 W) and South Orkney (60 58 0 53.4 00 S 43 26 0 42.6 00 W) (sub-Antarctic and Antarctic waters), currently housed at the Museo de La Plata (MLP), Argentina. Shells were measured according to the parameters and ratios indicated by Kamenev (2014), by using an ocular micrometer mounted on a stereoscopic microscope, and a caliper for larger animals. Text abbreviations are: A, length anterior to beaks; H, maximum height perpendicular to L; L, maximum shell length; W, shell width across closed specimens. ...
... This set of characters does not match those present in "Leda" rhytida, which has delicate, very posteriorly elongated shells, with a long internal ridge extending from the umbo to posterior margin, and bearing a large internal ligament and differentiated lamellar teeth near the beak. In contrast, the characters present in "Leda" rhytida are in agreement with those present in Leda ensicula Angas, 1877, the type species of Propeleda Iredale, 1924(Huber, 2010Kamenev, 2014). Consequently, "Leda" rhytida is here reallocated into Propeleda. ...
... Consequently, "Leda" rhytida is here reallocated into Propeleda. The distinction of Propeleda from other nuculanid and siliculid genera was discussed by Huber (2010) and Kamenev (2014). ...
In 1908, Dall described “Leda” rhytida as occurring off Acapulco, Mexico. The species was never figured, and has no subsequent published records. Despite that, it appears mentioned by several authors in check-lists. The identity of this species appears as controversial in the literature: some authors emended the type locality to either eastern South America or Chile, and the species was regarded either as a possible synonym of Propeleda longicaudata (Thiele, 1912), or considered as a nomen dubium. The aims of this study are to determine the actual provenance of the type material, to provide a proper redescription of the species, and to revise its generic placement and current status. The study of the syntypes and additional material from museum collections reveals that the species actually comes from southern Chile. Based on the elongated shell outline and distinctive hinge morphology, the species is here reallocated into Propeleda. Comparison of this species with P. longicaudata establishes that P. rhytida is a distinct and valid species.
... The studies were focused on the composition and distribution of the benthic fauna in the deep-sea basins of the Seas of Japan and Okhotsk, the Kuril-Kamchatka Trench, and at the abyssal plain of the Pacific Ocean adjacent to the Kuril-Kamchatka Trench (Malyutina & Brandt, 2013;Malyutina, Chernyshev & Brandt, 2018;Brandt et al., 2019). Benthic macrofauna rich in species number and abundance was found in these deep-sea ecosystems, with bivalves being one of the dominant groups of animals (Brandt et al., , 2020Kamenev, 2013Kamenev, , 2014Kamenev, , 2015Kamenev, , 2018aKamenev, , 2018bKamenev, , 2018cKamenev, , 2019Kamenev et al., 2022). Among the bivalve fauna of all the studied deep-sea areas of the northwestern Pacific Ocean, the Thyasiridae was the most species-rich family and many thyasirids were the dominant species in terms of abundance in the benthic macrofaunal communities (Kamenev, 2013(Kamenev, , 2015(Kamenev, , 2018a(Kamenev, , 2019Kamenev et al., 2022). ...
The Thyasiridae is one of the most species-rich families of bivalves in the deep-sea areas of the northern Pacific Ocean. Many thyasirid species form abundant populations in these regions and play an important role in the functioning of deep-sea benthic communities. However, most of these deep-sea thyasirid species have not been identified and many of them are new to science. Based on the material of bivalves collected by eight deep-sea expeditions in the northern Pacific Ocean during the period from 1954 to 2016, three new species of the genus Axinulus ( Axinulus krylovae sp. nov., A. alatus sp. nov., and A. cristatus sp. nov.) are described from the Kuril-Kamchatka and Japan trenches, the Bering Sea, and other deep-water regions of the northern Pacific Ocean (3,200–9,583 m depth). The new species are distinguished due to a unique and complex sculpture of the prodissoconch, including tubercles and numerous thin folds of varying length and shape, as well as due to a thickening of the shell in the adductor scar areas, thus rendering the scars raised above the inner surface of the shell. Comparisons with all species of the genus Axinulus are provided.
... Special focus on small-sized animals revealed many still undescribed taxa (e.g., Kamenev, 2013Kamenev, , 2014Kamenev, , 2015 Chernyshev et al., 2015;Downey and Janussen, 2015;Golovan, 2015;Jażdżewska, 2015;Krylova et al., 2015;Chaban et al., 2018;Golovan et al., 2019). Molecular data on macrostylid isopods suggested that for some abyssal benthic organisms, the oceanic trench may restrict gene flow and hence contribute to genetic structure amongst populations and ultimately diversification (Bober et al., 2018). ...
... The genus Propeleda Iredale, 1924 is known from off South America (Patagonia, Chile) (Güller and Zelaya 2015) and in the northwestern Pacific (Kamenev 2014). The material examined is very different from Propeleda loshka (Dall, 1908) (Panama to Peru) or P. extenuata (Dall, 1897) (Alaska to Panama) (Coan et al. 2000;Coan and Valentich-Scott 2012;under Nuculana). ...
Eighteen species of deep-sea bivalves were collected below the Oxygen Minimum Zone (OMZ) core off western Mexico. This material was obtained during the research cruises XVI, XVIB (off the northwestern Peninsula of Baja California), and VIII (Gulf of California) of the TALUD project aboard the R/V El Puma. The samples were taken with an epibenthic sledge (710–2077 m deep) and with a box corer (750–2600 m). The species belong to 10 families: Solemyidae, Nuculanidae, Malletiidae, Tindariidae, Limopsidae, Propeamussiidae, Tellinidae, Periplomatidae, Poromyidae and Lyonsiellidae. Environmental parameters (oxygen, salinity, sediments composition and temperature) are given for each sampling station. Tolerance to oxygen depletion is analysed for all deep-water species collected by the TALUD survey off the Baja California Peninsula. The additional file includes shell characteristics and distributional information for each species.
... It should be noted that the abyssal plain adjacent to the Kuril-Kamchatka Trench is characterized by high species richness of not only pectinoideans but also of the entire bivalve fauna. In total, 58 species of bivalves were found in the region, and many of them are new to science (Kamenev 2014(Kamenev , 2015Krylova et al. 2015;Kamenev 2016). It is known that one of the main environmental factors that control the occurrence and evolution of the bivalves in the deep-sea is food supply (Allen 2008). ...
Based on the material collected from 1949 to 2015 by nine deep-sea expeditions, four new species of the family Propeamussiidae (Mollusca: Bivalvia) and Catillopecten squamiformis (Bernard, 1978) were found in the abyssal zone (2,901-5,680 m depths) of the Bering Sea, the Sea of Okhotsk, and the northwestern Pacific. Parvamussium pacificum sp. nov., Catillopecten brandtae sp. nov., Catillopecten malyutinae sp. nov., and Catillopecten natalyae sp. nov. are described from the abyssal plain (4,860-5,680 m depths) adjacent to the Kuril-Kamchatka and Japan trenches (Pacific Ocean). Catillopecten squamiformis was found in the Bering Sea, the Sea of Okhotsk, and at the abyssal plain near the southeastern coast of Kamchatka and Aleutian Trench (Pacific Ocean) at 2,901-5,020 m depth. This is the first record of C. squamiformis from the northwestern Pacific. An expanded description of C. squamiformis with new data on its morphology and geographic distribution is given. To date, 12 species of the genus Catillopecten are known from the different regions of the World Ocean. A table with the main differences among all known species in the genus is provided.
... Starting in 2010, investigations of the deep-sea fauna of this extensive region have been continued by joint expeditions of Russian and German researchers (Malyutina & Brandt, 2013;. As a result of these studies, a rich bottom fauna of the abyssal and hadal zones of the North-western Pacific has been discovered and a large number of species of bivalves have been described (Filatova, 1958(Filatova, , 1960(Filatova, , 1971(Filatova, , 1976Ivanova, 1977;Filatova & Schileyko, 1984, 1985Krylova, 1993Krylova, , 1995Krylova, , 1997Kamenev, 2014Kamenev, , 2015Krylova et al., 2015). Representatives of the order Pectinida have also been found in deep-water basins of the Sea of Japan, the Sea of Okhotsk, the Bering Sea, and on the abyssal plains and in oceanic trenches of the North-western Pacific at depths to 8100 m. ...
Three new species of the genus Hyalopecten (Bivalvia: Pectinidae) from the abyssal and hadal zones of the Northwestern Pacific Ocean gennady m. kamenev A.V. Zhirmunsky Institute of Marine Biology of the Far Eastern Branch of the Russian Academy of Sciences, Palchevskogo St., 17, Vladivostok 690041, Russia Three new species, Hyalopecten vityazi sp. nov., H. abyssalis sp. nov. and H. kurilensis sp. nov., are described from the abyssal and hadal zones of the Northwestern Pacific. Hyalopecten vityazi was found in the Kuril-Kamchatka and Aleutian trenches at 6090– 8100 m depth. It is the most deep-water species of the order Pectinida. Hyalopecten abyssalis and H. kurilensis were found at the abyssal plain adjacent to the Kuril-Kamchatka and Aleutian trenches at 4550 –5045 m depth. To date, 13 species of the genus Hyalopecten are known from different regions of the World Ocean. A table with the main differences among all known species in the genus is provided.
A new species of deep-sea thyasirid (Bivalvia: Thyasiridae), Ascetoaxinus ravichandrani sp. nov., is described
and illustrated from material collected in the northern Indian Ocean during the FORV Sagar Sampada deep-sea cruise
no. 404. Specimens were collected from a depth between 1007 m and 1038 m off Kanyakumari, Gulf of Mannar, Tamil
Nadu, South India. This is the first record of the genus Ascetoaxinus from the Indo-West Pacific. The new species is
compared to A. quatsinoensis P.G. Oliver & Frey, 2014 from the Northeast Pacific and A. ovoideus (Dall, 1890) from
the Northwest Atlantic. It can be distinguished from these species by its external shell characters. Further comparisons
are made between the new species and other larger thyasirids recorded from deep waters around India, notably
Channelaxinus investigatoris E.A. Smith, 1895. The distinctions and similarities among these species are discussed,
highlighting the key features that set A. ravichandrani apart from related species.
The classification of the subclass Protobranchia followed here considers the families Nuculidae, Nuculanidae, Siliculidae, Sareptidae, Malletiidae, and Tindariidae in the order Nuculoida, and the family Acharacidae in the order Solemyoida. We also follow the separatation of Malletiidae from Nuculanidae, further justifying it on anatomical grounds. Fifteen protobranch species of Recent distribution in continental Chile, five Antarctic, and seven fossil species were studied. Out of a total of 35 species cited for the coast of Chile and Graham Land in Antarctica, only the following 27 are here accepted as valid: Nucula austrobenthalis Dell, 1990; Nucula (Nucula) falklandica Preston, 1912; Nucula (Nucula) fernandensis Villarroel, 1971; Nucula (Nucula) interflucta Marincovich, 1973; Nucula (Nucula) pisum Sowerby I, 1833; Ennucula eltanini Dell, 1990; Ennucula grayi (d'Orbigny, 1846); Ennucula puelcha (d'Orbigny, 1842); Nuculana (Saccella) cuneata (Sowerby I, 1833); Nuculana (Borissia) inaequisculpta (Lamy, 1906); Propeleda longicaudafaThiele, 1912; Tindariopsis sulcuta (Gould, 1852); Silicula patagonica Dall, 1908; Silicula rouchi Lamy, 1911; Yoldia (Aequiyoldia) eightsi (Jay, 1839); Yoldiella chilenica (Dall, 1908); Yoldiella ecaudata (Pelseneer, 1903); Yoldiella granula (Dall, 1908); Yoldiella indolens (Dall, 1908); Malletia chilensis des Moulins, 1832; Malletia magellanica Smith, 1875; Malletia patagonica Mabille & Rochebrune, 1889; Malletia inaequalis Dall, 1908; Malletiella sorror Soot Ryen, 1959; Tindaria virens (Dall, 1890); Tindaria salaria Dall, 1908; and Acharax macrodactyla (Mabille & Rochebrune, 1889). Nucula (N.) pseudoexigua Villarroel & Stuardo, a new species from the Strait of Magellan is described. The Antarctic species Propeleda longicaudata Thiele, 1912, is reported for the first time from the Strait of Magellan. A diagnosis for every taxonomic category and a detailed description for every species are given, including the shell features traditionally utilized, as well as the soft parts. Most features having so far been studied only on a few species of the subclass, allow a comparative analysis with the Chilean representatives, summarized as follows. (1) Size varies within the different families. Living nuculids are in general smaller than nuculanaceans and solemyaceans. The largest size found in the Chilean Nuculidae reaches 20.6 mm length in Ennucula grayi, whereas among Nuculanidae and Malletiidae, a maximum measured length of 51.0 mm was found in Malletia chilensis. A Chilean fossil of this genus measured 60 mm. (2) The studied species fall within the three known basic forms: nuculoid, nuculanoid, and solemyoid (Fig. 61). (3) No comprehensive study of hinge tendencies within each family has been attempted; such study would possibly permit to examine affinities and divergencies at lower taxonomic ranks. (4) The study of the ligament in specific taxa should be used to test the validity of prevailing models and interpretations. So far, the study of the ligament in Nuculidae and Nuculanidae has followed Owen's (1959) interpretation of an external or lamellar layer connected with the mantle margins, and another internal, fibrous layer connected to the isthmus of the mantle. A resilifer or chondrophore interrupts the two teeth series, and is directed anteriorly in Nucula and Ennucula (Fig. 3A, cdr), is more or less straight in Yoldia (Fig. 129), and is directed posteriorly in Nuculana (Fig. 3). Previously, Stempell (1898a) demonstrated that the ligament in Malletia can be divided in anterior, central and posterior parts, the central part corresponding to the resilium (inner layer of the ligament), and the anterior and posterior parts with an external origin. Thus, such similar differentiation in Nuculidae, Nuculanidae and Malletiidae, suggested that the resilium of internal position in Nucula and Nuculana, had migrated to become external in Malletiidae, without dissapearing. An intermediate stage in its position is observed in Tindariopsis, as was shown by Stempell (1898a). Relevance is given to the novel and most stimulating interpretation on the evolution of the ligament in the bivalves advanced by Waller (1990). He questions the traditional model of an amphidetic primary ligament of three layers, and proposes a protobranch stem group from which two major types of ligament for the bivalves evolved. (5) Variation in number and size of hinge teeth does not allow to use them as taxonomic features of generic or suprageneric value, but size and form may sometimes offer specific taxonomic value, as noted by Knudsen (1970) and Villarroel (1971). (6) The palps are very similar in the Nuculacea and Nuculanacea, but their homology with the Solemyidae is not well known. The palps in Solemya are not interpreted as doubled palps appendixes of other protobranchs; the palps sheets would be reduced to simple ridges (Fig. 11) in the edge of the furrow that joins the mouth with the appendixes (Figs. 15-17)(Ridewood, 1903; Morse, 1913; Yonge, 1939; Reid, 1980). The appendix or palp tentacle on the external sheet of every palp considered by Drew (1901) be equivalent of a pair of hypertrophiated fold (Figs. 10, 12, other figs., tp) differ in position according to family (Fig. 61). In the Nuculanacea, the palp appendix is located on the terminal portion of the external palp sheet (e.g., Fig. 5, Silicula rouchi; Fig. 77, Nuculana (S.) cuneata; Fig. 90, Nuculana (B.) inaequisculpta). In the Nuculidae, the palp appendix is displaced to the end, because behind it there is an additional, non-extensible structure termed the "palp caecum," which represents a pair of hypertrophiated folds (Stasek, 1965). The proximal end of the palp appendix is linked with the surface of the palp external sheets and the palp caecum (Fig. 10, bp); its musculature is fused with the posterior foot retractor. Stasek's (1961) observation of this feature in Acila was corroborated, without exception, in every species studied. Nevertheless, in almost all the cases, the appendix was found in different degrees of contraction, preventing recognition of specific differences (Figs. 4 and 62, 74 and 76, tp). (7) In a general comparative analysis, the value of the soft parts in the differentiation of the higher categories within the subclass, is corroborated; however, due to the limited available knowledge of many internal structures and the few studied species, their taxonomic role at the specific level cannot be always ascertained. On the other hand, the complexity observed in some of the internal morphological parts permitted us to set forth complementary interpretations on their possible phylogenetic value, particularly in the case of the stomach, the position of the heart, and the configuration of the various types of siphons. Although stomach morphology has been described for species of Nucula, Nuculana and Malletia, a comparison became necessary, resulting in the identification of a new caecum and changes in the interpretation of the features observed by previous authors. In fact, its study in the available species allowed the conclusion that there is not one basic type or "Gastroproteia," as proposed by Purchon (1956, 1959), but three. These are: Type Ia. Common to the genera Nucula and Ennucula and characterized by several (three or four) ciliary sorting areas and a wide extension of the typhlosole (Figs. 18-32, 60). Type Ib. Common to the genera of Nuculanidae and Malletiidae and characterized by three ciliary sorting areas and a small extension of the minor typhlosole (Figs. 33-56, 60). Type Ic. Common to the genera of Solemyidae and Nucinellidae and characterized by the absence of distinct sorting areas and lack of typhlosoles. It is not difficult to differentiate the internal and external features recognized in the stomach of Nuculacea and Nuculanacea. The dorsal hood is smaller in Nuculanacea than in Nuculacea, and the three ducts that communicate the stomach with the digestive diverticula are also different in these two superfamilies (Figs. 18-56, 60). Similar differences were found in the ciliary sorting areas and the number of folds. For instance, the three additional sorting areas as1, as2 and as3 described by Purchon (1956), although not present in all species, can also be used in interspecific differentiation. Thus, the first one was found only in Nucula (Nucula) pisum (Fig. 21) and Ennucula puelcha (Figs. 29-31), but not in the other studied species of these genera; the second sorting area was found presenting different sizes in Nucula (Nucula) pisum, Nucula (Nucula) fernandensis, and Ennucula puelcha, being largest in the latter. The third above named sorting area was not observed in the studied Nuculacea, and none were found in the studied Nuculanacea. Such differences do not back Purchon's (1987b) generalization that one description can embody all of them (Fig. 60). Undoubtedly, the complexity of the gastric shield with its biggest modification in Propeleda and Malletia is larger in Nuculanacea than in Nuculidae and Solemyidae, but presently it is difficult to establish generic or specific differences. On the other hand, folding of the typhlosoles entering the style-sac has shown specific constancy in the studied species of Nucula and Ennucula. Development of the typhlosoles in Nuculanacea shows a different pattern. (8) Attention has also been given to the number of loops observed in the gastric and medium intestine with a pattern of coiling, which according to Heath (1937) is specific, with minimal intraspecific variation as observed in Nucula (Nucula) pisum and Nucula (Nucula) pseudoexigua (Figs. 63-67). It begins on the side of the stomach and continues anteriorly in some species almost reaching the mouth. It turns then dorsally to the esophagus and continues posteriorly above the stomach, or continues ventrally to form the coils prior to its final turn backwards.