DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



21 



Fig. 16 Yoldiella similiris. Lateral views of a series of shells to 

 show changes in shape with growth. Specimens from Sta. 236 

 Argentine Basin. (Scale =1.0 mm). 



Fig. 15 Yoldiella spp. Dorsal 

 views of shells of a, Y. obesa 

 Q obesa; b, Y. similiris and c, Y. 



lucida. (Scale = 1.0 mm). 



is particularly noticeable in the larger specimens. The height 

 to length ratio is greater in Y. similiris. 



Internal morphology (Fig. 14). The internal morphology 

 is very similar to that of Y. lucida. The adductor muscles are 

 unequal in size, the posterior muscle is oval and approxi- 

 mately 1/3 the size of the anterior. There are a smaller 

 number of gill filaments (maximum of 11 observed) and the 

 labial palp ridges vary between 11-13. The stomach and style 

 sac appear relatively smaller than those in Y. lucida and the 

 hind gut, while making a single loop on the right side of the 

 body, is not as deep as it is in the latter species. A maturing 

 female (3.4 mm total length) had 203 ova with a maximum 



50 



W/L 



30 

 80 



60 

 60 

 50H 



H/L 



PL/TL 



12 3 4 



Length (mm) 



Fig. 17 Yoldiella similiris. Variation in the ratios of height H/L, 

 width W/L and postero-umbonal length to length against length of 

 a subsample from Sta. 236. Argentine Basin. 



