62 



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



Distributed in the north eastern Atlantic, Bay of Biscay, off 

 northwest Africa and the Azores at abyssal rise depths. 

 Depth range: 1354-3301 m. 



Shell description (Figs. 95-98). Shell ovate, moderately 

 inflated, smooth with very fine concentric lines near ventral 

 margin forming slight ridges in larger specimens, occasionally a 

 few faint radial lines are present in larger specimens; perios- 

 tracum pale straw-coloured, slightly iridescent; umbos very 

 slightly anterior of midline, moderately inflated; proximal dor- 

 sal margin straight or slightly concave on either side of umbo, 

 antero-dorsal and anterior margin merge in an even curve in 

 most specimens, distally postero-dorsal margin slopes down to 

 posterior margin; posterior margin with slight angulation, poste- 

 rior limit of anterior and posterior margins dorsal to horizontal 

 mid-plane, ventral margin a smooth curve; hinge plate relatively 

 narrow, barely extending to outer margins of adductor muscles, 

 anterior hinge line relatively straight, distally slopes away from 

 dorsal shell margin, chevron-shaped teeth small, posterior hinge 

 line slightly curved anteriorly, with same number of teeth in 

 each series (8/8 in a specimen 2.2 mm and 12/12 in a specimen 

 3.5 mm); ligament amphidetic, small, rounded, extends below 

 hinge plate. 



Prodissoconch length: c. 190 p-m. Maximum recorded shell 

 length: 3.65 mm. 



There is a slight increase in the post-umbonal length as 

 length increases, otherwise the ratios of height/length and 

 width/length remain constant. 



Internal morphology (Fig. 99). The mantle edge is well- 

 developed, particularly the inner muscular fold which at its 

 dorsal edge, contains secretory cells beneath the main rejec- 

 tion tract. Postero-ventral to the siphons, both the middle 

 and inner folds are increased in size, convoluted and heavily 

 ciliated and form a feeding aperture. Exhalent and inhalent 

 siphons are combined, the latter being open ventrally. A pair 

 of lateral haemocoelic canals are present the junction 



50 

 40 

 80 



w\ L • * >*;3$r,. 



• „ • 



H\L 



*• • • •• • 



60 

 60 

 50 



PL\TL 



• • • v9*. • 



2 3 4 



Length (mm) 



Fig. 98 Yoldiella insculpta. Variation in the ratios of height H/L, 

 width W/L and postero-umbonal length PT/TL to length against 

 length of a sample from Sta. BG IV DS 62, West European 

 Basin. 



Fig. 99 Yoldiella insculpta. Lateral views from the right and left 

 sides of the internal morphology of a specimen from Sta. S 65, 

 West European Basin. (Scale = 1.0 mm). For identification of 

 parts see Fig. 34. 



between the siphons. The siphonal tentacle is large and found 

 equally to either the right or left side of the siphonal 

 embayment. There is a well-developed anterior sense organ. 

 Adductor muscles are slightly unequal in size, the anterior 

 being the larger. 



Gill plates are relatively small and difficult to count in 

 preserved specimens (maximum recorded c. 18-20). The 

 labial palps are well-developed with approximately up to 26 

 closely spaced ridges on each inner face. The palps extend 

 between 1/3-1/2 across the body and each bears a long thin 

 palp proboscis. The mouth is positioned a short distance 

 posterior to the anterior adductor muscle. The foot is rela- 

 tively large with a broad sole with a small papilla at the 

 posterior limit. There is a large byssal gland in the heel (Fig. 

 100). The arrangement of pedal retractor muscles is similar to 

 that of other species of Yoldiella. The cerebral ganglia are 

 relatively large, the visceral ganglia are small and elongate. 

 The pedal ganglia lie ventral to the style sac in the proximal 

 part of the foot and are moderately large. The stomach is 

 displaced slightly to the right of the body and is of moderate 

 size with a large style sac. The hind gut configuration is 

 unique. There are three loops to the right of the body and two 



