DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



53 



Fig. 79 Yoldiella amerkana. Lateral view of shell from the right 

 side, lateral internal view of a left valve, enlarged view of a 

 hinge-plate of a left valve and a dorsal view of the shells. 

 Specimens from Sta. 80, North America Basin. (Scale = 1.0 mm). 



50 



30 

 80 



W/L . . 



H/L ' . * 





60 J 

 60 



50 



PL/TL 





1 2 3 



Length (mm) 



Fig. 80 Yoldiella americana. Variation in ratio of height H/L, 

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

 length of subsamples from Sta. Biovema DS 05, Cape Verde 

 Basin (closed circles) and from Sta. 247 Argentine Basin (open 

 triangles). 



posterior margin rounded or with slight angulation forming 

 slight subrostrum; hinge plate moderately strong, long strong 

 chevron-shaped teeth, equal numbers on anterior and poste- 

 rior plates, 14/14 in a specimen 2.44 mm total length; 

 ligament amphidetic, small, rectangular in shape, small poste- 



rior external extension of fused periostracum. 



Prodissoconch length: 187 u.m. Maximum recorded shell 

 length: 2.5 mm. 



There is little change in the posterior umbonal length/total 

 length ratio or the height/length ratio with increasing size, 

 however, individuals tend to become more tumid with 

 increasing length. The width/length ratio is the most variable 

 feature when comparing populations from different areas 

 (Fig. 80). Thus, specimens from the west Atlantic are rela- 

 tively wider than those from the the east Atlantic although in 

 lateral view (Fig. 81) and in internal anatomy they differ 

 little. Populations are remarkably different in their size 

 range. This is probably indicative of single massive successful 

 spatfalls at different times (Fig. 82). 



Internal morphology (Fig. 83). The anterior sense organ 

 is moderately well-developed. The siphons are combined, 

 with the inhalent siphon open at the ventral edge. A siphonal 

 tentacle originates from the base of the siphonal embayment, 

 either on the right or the left side. There is a feeding aperture 

 ventral to the siphons. Anterior to this the inner muscular 

 lobe is enlarged convoluted and heavily ciliated. The adduc- 



Fig. 81 Yoldiella americana. Outlines of shells from the right side 

 to show change in shape with growth of specimens from Sta. 

 Biovema DS 05 Cape Verde Basin (left) compared with 

 specimens from Sta. 247 Argentine Basin. (Scale = 1.0 mm). 



