. Bulletin of the Natural History Museum Zoology. 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 = mm). 50 30 80 W/L . H/L ' . * 60J 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 circl
. Bulletin of the Natural History Museum Zoology. 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 = mm). 50 30 80 W/L . H/L ' . * 60J 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 mm total length; ligament amphidetic, small, rectangular in shape, small poste- rior external extension of fused periostracum. Prodissoconch length: 187 Maximum recorded shell length: 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 feedin
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