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. Bulletin of the Natural History Museum Zoology. DEEP-SEA PROTOBRANCHIA (BIVALVIA) 17 50 30 80 W/L ..• V %. 60 H/L • • * • • • •• • • • • ••• • •» 60 50 PL/TL ':\ .°- * —I- 7 Length (mm) Fig. 7 Yoldietla 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 urn. Maximum recorded shell length: 4.45 mm.. Fig. 8 Yoldiella obesa obesa. Neotype: USNM No. 38419. Lateral view o