24 



J.A. ALLEN, H.L. SANDERS AND F. HANNAH 



50 



30 

 80 



60 

 60 

 50 



W/L 



H/L 



PL/TL 



1 2 3 



Length (mm) 



Fig. 23 Yoldiella capensis. Variation in the ratios of height H/L, 

 widthW/L, and postero-umbonal length PL/TL to length against 

 length of a subsample of specimens from Sta. 188 Cape Basin. 



Fig. 24 Yoldiella capensis. Lateral view from the right side of the 

 internal morphology. Specimen from Sta. 188, Cape Basin. (Scale 

 = 1.0 mm). For identification of the parts see Fig. 5. 



them. The foot is large, well-developed with an anteriorly 

 directed attitude. The stomach and style sac are also large. 

 The hind gut forms a single loop on the right side of the body. 

 This species bears some resemblance to Yoldiella lucida and 

 to Y. bilanta (Fig. 137). 



Yoldiella bilanta (new species) 



Type locality. R.V. Atlantis II, Cruise 42, Sta. 192, Cape 

 Basin, 17.5.1968, 23°05.0'S, 12°31.5'E, Epibenthic Trawl, 

 2117-2154 m. 



Type specimen. Holotype: BM(NH) 1992027, Paratypes: in 

 collection held by J.A. Allen. 



Material. 



Cruise Sta Depth No Lat Long Gear Date 



(m) 



CAPE VERDE BASIN 



Atlantis II 138 1944- 1 10°36.0'N 17°52.0"W ES 4.2.67 

 31 1976 



141 2131 2 10°30.0'N 17°51.5'W ES 5.2.67 



142 1624- 1 10°32.0'N 17°51.5'W ES 5.2.67 

 1796 



144 2051- 22 10°36.0'N 17°49.0'W ES 5.2.67 

 2357 



CAPE BASIN 



Atlantis II 191 1546- 1025 23°05.3'S 12°31.5'E ES 17.5.68 

 42 1559 



192 2117- 1697 23°02.0'S 12°19.0'E ES 17.5.68 

 2154 



Found in the Cape Verde and Cape Basins at lower slope 

 depths. Depth range: 1446-2357 metres. 



Shell description (Fig. 25). Shell slender, elongate ovate, 

 inequilateral, smooth with few fine concentric lines, perios- 

 tracum pale straw-coloured; umbos anterior to midline, 

 slightly inflated, orthogyrate; antero-dorsal margin moderat- 

 ley convex, curves evenly to anterior margin, ventral margin 

 long, smooth curve, anterior curvature slightly more convex 

 than posterior, posterior margin narrow, slightly produced 

 with supramedial rounded angle, postero-dorsal margin 

 slopes gradually more or less straight or slightly concave from 

 umbo to posterior limit of hinge plate, then more acutely to 

 posterior margin; hinge plate, strong, long, relatively narrow 

 and straight with 9 anterior and 10 posterior teeth in largest 

 specimen; anterior and posterior plates approach margin 

 below umbo; ligament amphidetic, bilobed goblet-shaped, 

 extends below hinge line, short anterior and posterior exter- 

 nal extensions of fused periostracum. 



Prodissoconch length c 190 p,m: Maximum recorded shell 

 length: 8.0 mm. 



The morphology of the shell is similar to that of Yoldiella 

 capensis (Fig. 21). With increasing size the shell becomes 

 more posteriorly elongate and the postero-dorsal margin 

 becomes more straight so that the slight angulation at the 

 level of the limit of the posterior hinge margin becomes much 

 less obvious (Figs. 25 & 26). There is little change with 

 growth in the height and length and width to length ratios. 

 The hinge plate is more narrow and teeth less robust than in 

 Y. capensis and the lateral 'dumb bell' outline of the ligament 

 is very different from the more rectangular and deeper 

 ligament of the latter species. 



Internal morphology (Fig. 27). The most conspicuous 

 difference from Y. capensis is the size and shape of the 

 adductor muscles which are larger and more rounded in 

 Yoldiella bilanta The labial palps are relatively small and 

 extend approximately 1/3 distance across body. They bear up 

 to 16 palp ridges. The gill plates are relatively narrow and 

 number up to 18. The single loop of the hind gut has a greater 

 diameter but is not as ventrally deep as that in Yoldiella 

 capensis. Similar features to Yoldiella capensis include well- 

 developed combined siphons, nervous system, and a large, 



