DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



17 



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W/L 



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60 J 





H/L 



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PL/TL 



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Length (mm) 



Fig. 7 Yoldiella lucida. Comparison of the shell proportions of three populations; small closed circles. Gulf of Maine USNM 202847; large 

 closed circles, a subsample from Thor' Sta. 28; open circles, a subsample from Sta. 105. H/L height/length, PL/TL postero-umbonal 

 length/total length, W/L width/length - all against length. 



Also examined: 



U.S. Fish Comm. USNM 73163 labelled Y. lucida (Loven), 

 Cisco Bay 1873 m; Station 2072 USNM 38419 labelled 

 Yoldiella inflata (young), off Georges Bank 1569 m; 

 USNM 35189 labelled Yoldiella inflata off Georges Bank 

 2360 m. 



Restricted to the North American Basin at lower slope 

 depths. Depth range 1254-2886 metres. 



Shell description (Figs. 8-9 & 15). Shell moderately frag- 

 ile, elongate, ovate, with fine concentric ridges, inequilateral; 

 umbo moderately inflated, anterior, inwardly directed; dorsal 

 margin in the region of the umbos in all but a few specimens, 

 slightly concave, dorsal margins form sharp raised edge, 

 antero-dorsal margin smooth curve joining anterior margin 

 with no angulation, ventral margin smooth, relatively deep 

 curve, postero-dorsal margin in large specimens almost 

 straight, subangulate with posterior margin, extreme poste- 

 rior limit of shell dorsal to horizontal midline; hinge plate 

 moderately strong, parallel to margin, chevron-shaped teeth 

 fairly stout, anterior and posterior hinge plates with same 

 number of teeth, occasionally with one additional tooth on 

 posterior plate; ligament amphidetic, circular in lateral view, 

 extends ventral to hinge plate but without chondrophore, 

 short anterior and posterior secondary external periostracal 

 extensions. 



Prodissoconch length: 215 u,m. Maximum recorded shell 

 length: 4.45 mm. 



Fig. 8 Yoldiella obesa obesa. Neotype: USNM No. 38419. Lateral 

 view of a right valve and a hinge plate of left valve, also a dorsal 

 view of the umbos showing the outline of the prodissoconch. 

 (Scale = 1.0 mm). 



