56 



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



Fig. 85 Yoldiella subcircularis . Outlines of shells from the right 

 side to show variation in shape. Specimens taken from Sta. BG 

 VI DS 76, West European Basin. (Scale = 1.0 mm). 



50 

 40 



W\L 



■ • X. ■ "j 



America specimens. In contrast the small number of speci- 

 mens from the Guyana Basin differ slightly in having a lower 

 height/length ratio (significant at the 0.1 level). 



Internal morphology (Figs. 87 & 88). In Y. subcircularis 

 the volume of the mantle cavity is large compared with that of 

 the body. A well-developed sense organ lies at the far 

 anterior margin. The siphons are combined and the inhalent 

 siphon is shorter than the exhalent and is open ventrally. 

 Both siphons are thin-walled and a pair of lateral haemoco- 

 eles are present in the junction between them. There is a 

 moderately large siphonal tentacle, most frequently found on 

 the left side. The feeding aperture is well-developed with 

 areas of glandular mantle epithelium on each side which are 

 probably extended in life as paired, broad flaps. For some 

 distance anterior to these, the inner muscular lobe is particu- 

 larly well-ciliated and it may be that the feeding area either is 

 separated from the pedal gape by a ciliary junction or it is the 

 region where pseudofaeces are formed. The adductor 

 muscles are small, approximately equal in size, oblong in 

 shape and lie close to the dorsal and anterior and posterior 

 mantle margins respectively. 



The gill plates are relatively few in number (18 in a 

 specimen 3.0 mm). Tissue junctions join the demibranchs to 

 the mantle dorsally. The gill plates are particularly muscular 



i 



80 

 70 



*j.A::V'i£ *■ '■ 



H\L 



60- 

 50- 



PL\TL 



■ 



••I"..* 



• 

 • 



• 



:.-.y*-. 



*■■.. 



■ 

 ■ 



■ 





i 





i 







i 





1 





Length(mm) 



Fig. 86 Yoldiella subcircularis. Variation in ratios of height H/L, 

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

 length of subsamples from Sta. BG DS 76, West European Basin 

 (closed circles) and Sta. Walvis QS 07, Cape Basin (closed 

 squares). 



Fig. 87 Yoldiella subcircularis. Lateral views from right and left 

 sides of the internal morphology of a specimen from Sta. 80 North 

 America Basin. (Scale =1.0 mm). For identification of parts see 

 Fig. 34. 



