DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



55 





WS10 4354 



10 



47°27.3'N 

 - 47°18.2'N 



9°39.9'W 



WS 



11. 8.76 



Jean Charcot DS 11 4823 



1 



48°18.8'N 



15°11.5'W 



DS 



1. 8.76 



(Incal) 







- 48°18.6'N 



15°12.0"W 









DS14 4254- 



3 



47°32.6'N 



9°35.7'W 



DS 



7. 8.76 





4248 





- 47°32.6'N 



9°35.1'W 









DS15 4268 



12+2v 47°29.8'N 



9°33.4'W 



DS 



9. 8.76 









- 47°30.3'N 



9°33.4'W 









CPU 4823 



1 



48°20.4'N 

 - 48°21.2'N 



15°14.6'W 

 15°13.7"W 



CP 



1. 8.76 



Chain 106 



330 4632 



8 



50°43.5'N 

 - 50°43.4'N 



51°07.0"W 

 51°09.0"W 



ES 



29. 8.72 



CANARIES BASIN 













Discovery 



6714 3301 



3 



27°13.0'N 



15°41.0"W 



ES 



20. 3.68 



GUYANA BASIN 













Knorr 25 



287 4980- 

 4934 



5 



13°16.0'N 



54°52.2'W 



ES 



24. 2.72 





288 4417- 



64 



11°02.2'N 



55°05.5'W 



ES 



25. 2.72 





4429 













Jean Charcot DS03 



1 



10°47.1'N 



42°40.7'W 



DS 



16.11.77 



(Biovema) 







- 10°47.1'N 



42°40.3'W 









DS05 5100 



6 



10°45.9'N 

 - 10°46.0'N 



42°40.2'W 

 42°39.8'W 



DS 



18.11.77 





CP04 5100 



1 



10°45.9'N 

 - 10°45.9'N 



42°40.2'W 

 42°39.3'W 



CP 



18.11.77 



CAPE BASIN 













Jean Charcot DS05 4560 



3 



33°20.5'S 



2°34.0'E 



DS 



30.12.78 



(Walvis) 



DS06 4585 



7 



33°24.5'S 



2°32.0'W 



DS 



31.12.78 





DS07 5100 



20 



26°59.7'S 



1°07.1 'E 



DS 



1. 1.79 





DS08 5225 



3 



26°59.6'S 



1°07.3'E 



DS 



5. 1.79 





DS09 5220 



15 



26°59.9'S 



1°06.7'E 



DS 



6. 1.79 



NORTH AMERICA BASIN 











Atlantis II 



12 

 Chain 50 



70 4680 



1 



36°23.0'N 



67°58.9"W 



ET 



23. 8.64 



78 3828 



2 



38°00.0'N 



69°18.7'W 



ET 



30. 6.65 





80 4970 



45 



34°49.8'N 



66°34.0"W 



ET 



2. 7.65 





81 5042 



14 



34°41.0'N 



66°28.0"W 



ET 



2. 7.65 





83 5000 



2 



34°46.5'N 



66°30.0"W 



ET 



3. 7.65 





85 3834 



8 



37°59.2'N 



69°26.2'W 



ET 



5. 7.65 



Atlantis II 



17 

 Atlantis II 



92 4694 



3 



36°20.0'N 



67°56.0"W 



ET 



13.12.65 



93 5007 



23 



34°39.0'N 



66°26.0'W 



ET 



14.12.65 



24 



121 4800 



2 



35°50.0'N 



65°11.0"W 



ET 



21. 8.66 



Atlantis II 



175 4667- 



2 



36°36.0'N 



68°29.0'W 



ES 



29.11.67 



40 



4693 













This is a true abyssal species found only in the deepest part of 

 the Basins and, except for the Argentine & Norwegian Basins 

 is widely distributed throughout the Atlantic. Depth range: 

 3250-5225 m. 



Shell description (Figs. 84 & 85). Shell small, moderately 

 thick, oval, inflated, inequilateral, posterior margin slightly 

 produced; surface with very fine irregular concentric lines, 

 fine radial striae present in some specimens, dorsal area 

 between umbo and posterior margin furrowed; umbos raised 

 slightly, orthogyrate; dorsal margin proximally straight on 

 either side of umbo, distally antero-dorsal margin joins 

 rounded anterior margin in broad curve which continues to 

 ventral margin, more convex posteriorly, distal postero- 

 dorsal margin curves more gradually to produce slightly 

 drawn out posterior margin, limit of posterior margin supra- 

 medial; hinge plate moderately broad, long, slightly curved, 

 up to 10 small 'V-shaped, chevron teeth on each side of 

 ligament; ligament amphidetic, small, 'goblet'-shaped, small 

 external extension of fused periostracum on either side of 

 umbo, small chondrophore present. 



Prodissoconch length: 146-187 u.m. Maximum recorded 

 shell length 4.4 mm. 



The description of Portlandia (Y) subcircularis (Odhner, 

 1960) agrees well with that presented above. 



With increasing length, the posterior part of the shell 

 becomes slightly more extended and at the same time the 

 postero-dorsal margin becomes more smoothly curved (Figs. 

 85 & 86). The width to length ratio also increases with 

 increasing size, but little proportionate change occurs in the 

 height to length ratio although this varies in individuals 

 between 0.72 and 0.84. The height/length ratios are the 

 highest recorded for any species of Yoldiella. In some larger 

 specimens the dorsal margin immediately anterior and poste- 

 rior to the umbos may become slightly concave and as a result 

 a small, shallow lunule and escutcheon may be formed. 

 Comparison of specimens from the different Basins show that 

 northerly (W. European) and southerly (Angola) populations 

 in the eastern Atlantic are similar in form as too are the North 



Fig. 84 Yoldiella subcircularis . 

 Lateral view of a shell from the 

 right side and a right valve to show 

 the detail of the hinge-plate. 

 Specimens from Sta. 285 Guyana 

 Basin and Sta. 93 North Atlantic 

 Basin respectively. (Scale = 1.0 

 mm). 



