28 



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



Fig. 31 Yoldiella similis. Lateral view from the right side of the 

 internal morphology of a specimen from Sta. 197. Angola Basin. 

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



Type specimen. Holotype: BM(NH) 1992025, Paratypes: in 

 collection held by J. A. Allen. 



Material. 



Cruise 



Sta 



Depth No Lat 

 (m) 



Long 



Gear Date 



SURINAM BASIN 



Knorr 25 



299 1942- 54 

 2076 



7°55.1'N 55°42.0'W ES 29.2.72 



Restricted to one Station on the lower slope of the Surinam 

 Basin. Depth range: 1942-2076 m. 



Shell description (Figs. 32 & 33). Shell small, ovate, 

 moderately inflated, slightly inequilateral, surface smooth in 

 small individuals, ridges on ventral half of shell of larger 

 specimens, periostracum pale yellow; umbo small, slightly 

 raised, internally directed; no lunule or escutcheon; dorsal 

 margin convex, anterior and posterior margins slope at 

 similar angle from umbo, anterior margin smooth curve, 

 posterior margin slight extended and slightly subrostrate, 

 postero-dorsal margin slightly angled at posterior limit of 

 hinge plate, postero-ventral margin sinuous, ventral margin 

 slightly more convex posteriorly; hinge plate strong, rela- 

 tively long, with up to 9 anterior and 10 posterior chevron- 

 shaped teeth, hinge plate narrow below umbo; ligament 

 amphidetic, internal, 'goblet-shaped'. 



With increasing shell length, there is little change in the 

 height to length ratio but there is a slight increase in the width 

 to length ratio. There is also gradual post-umbonal extension 

 with the sinuous nature of the postero-ventral margin becom- 

 ing more conspicuous (Fig. 35). 



Prodissoconch length: 179 |xm. Maximum recorded shell 

 length: 3.69 mm. 



Internal morphology (Fig. 34). The combined siphons, 

 the siphonal tentacle (usually on the left side), and the 

 anterior sense organ are as in Y. similis. The adductor 



Fig. 32 Yoldiella sinuosa. Lateral view of a shell from the right 

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

 Sta. 299 Guyana Basin. (Scale = 1.0 mm). 



Fig. 33 Yoldiella sinuosa. Outline drawings of shells of different 

 sizes from the right side to show change in shape with growth. 

 Specimens from Sta. 299 Guyana Basin. (Scale = 1.0 mm). 



muscles are approximately oval, the anterior muscle being 

 the larger and being almost twice the size of the posterior. 

 The gills have up to 17 plates. The labial palps are moderate 

 in size and extend 1/3 way across the body. They have up to 

 15 moderately broad ridges and each bears a long palp 

 proboscis. The pedal ganglia are large, round, with large 

 statocysts dorsal to them. The cerebral ganglia are slightly 

 larger than the visceral, both are club-shaped and well- 

 developed. The foot is large with a large byssal gland. The 

 stomach is large with a small, narrow style sac. The hind gut 

 takes an 'S-shape' course to the right side of the body before 

 returning to the mid dorsal margin and thence to the anus. 

 There is a fine typhlosole present along the length of the hind 

 gut. Ingested material was seen in part of the left digestive 

 diverticulum. The kidney is well-developed, extending anteri- 



