DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



45 



i 



Fig. 66 Yoldiella inconspicua africana. Lateral view from the right 

 side of the internal morphology of a specimen from Sta. 8532 6 

 Cape Verde Basin. (Scale = 1.0 mm). For identification of parts 

 see Fig. 12. 



visceral ganglia are 'club'-shaped, the visceral being the 

 larger. The ovoid pedal ganglia lie central in the foot, just 

 dorsal and anterior to the byssal gland. A large statocyst lies 

 dorsal to each pedal ganglion. 



The mouth is set posterior to the anterior adductor muscle. 

 The oesophagus is relatively narrow while the stomach is 

 large with a small style sac which does not penetrate the foot. 

 The duct of the right digestive diverticulum passes dorsally 

 around the hind gut to join the stomach close to the oesoph- 

 ageal aperture. One of the two left diverticula opens ventral 

 to the gastric tooth while the second opens adjacent to that 

 from the right. No ingested material was observed in the 

 diverticula. From the style sac, the hind gut passes between 

 the pedal ganglia and the byssal gland before turning dor- 

 sally. It makes a very small flexure on the left side immedi- 

 ately before crossing to the right side at the dorsal limit of the 

 body posterior to the ligament. On the right side of the body 

 the hind gut makes a broad sweeping loop as far as the 

 posterior face of the anterior adductor before passing dorsally 

 and posteriorly to the anus. 



The gonads overlie the posterior, dorsal and dorso-lateral 

 sides of the viscera and extend antero-dorsally with part lying 

 centrally between the digestive diverticula. The kidney is 

 small in comparison to the other species of Yoldiella 

 described here, extending no further than the posterior edge 

 of stomach. The heart is relatively large. 



This subspecies differs from Y. inconspicua s.s. in having a 

 more elongate outline and a more pointed posterior shell 

 margin. The dorsal margin is slightly more convex, the 

 height/length ratio is slightly greater. The hind gut differs 

 slightly in the course taken to the right of the body (Figs. 60 & 

 66). 



Fig. 67 Yoldiella inconspicua profundorum. Lateral view of shell 

 from the right side and detail of the hinge-plate of a left valve. 

 Specimens from Sta. 242 Argentine Basin. (Scale = 1.0 mm). 



Yoldiella inconspicua profundorum (new subspecies) 



Type locality. R.V. Atlantis II, Cruise 60, Sta. 256, 

 Argentine Basin, 24.3.1971, 37°40.9'S, 51°19.3'W, 

 Epibenthic Trawl, 3906-3917 m. 



Type specimen. Holotype: BM(NH) 1992038, Paratypes: in 

 collection held by J. A. Allen. 



Material. 



Cruise Sta Depth No Lat Long Gear Date 



(m) 



ARGENTINE BASIN 



Atlantis II 242 4382- 50 38°16.9'S 51°56.1'W ES 13.3.71 

 60 4402 



243 3815- 1 37°36.8'S 52°23.6'W ES 14.3.71 



3822 

 245 2707 2 36°55.7'S 53°01.4"W ES 14.3.71 

 247 5208- 1 43°33.0'S 48°58.1"W ES 17.3.71 



5223 

 252 4435 23 38°29.8'S 52°09.1'W ES 22.3.71 

 256 3906- 19 37°40.9'S 5ri9.3"W ES 24.3.71 



3917 

 259 3305- 5 37°13.3'S 52°45.0'W ES 26.3.71 



3317 

 264 2041- 6 36°12.7'S 52°42.7'W ES 28.3.71 



2048 



Restricted to the Argentine Basin at lower slope and abyssal 

 depths. Depth range: 2041-5223 m. 



Shell description (Fig. 67). Shell small, equilateral, mod- 

 erately inflated, subovate, fine concentric lines; umbos 

 slightly raised, inwardly directed; shell outline somewhat 

 variable, anterior and posterior dorsal margins straight proxi- 

 mal to umbo, curve distally to broadly rounded anterior and 

 slightly pointed posterior margins, ventral margin smooth 

 curve; anterior and posterior hinge plates relatively narrow 



