DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



37 



Fig. 50 Yoldiellafrigida. Lateral 

 views of shells from the right side. 

 Specimens a & b from Franz Joseph 

 Fjord, E. Greenland; c, from Sta. 

 INCAL DS 04 Bay of Biscay; d, from 

 off Martha's Vinyard (USNM No. 

 193343). (Scale = 1.0 mm). 



approximately equilateral (postumbonal length 45-50% of 

 total length), smooth, moderately thin, pale yellow irrides- 

 cent periostracum, with banding patterns, narrow dark bands 

 and broader light bands between; umbo slightly raised 

 directed somewhat posteriorly; dorsal margins proximally 

 straight, distally curving to anterior and posterior margins, 

 posterior and anterior dorsal margins very slightly angulate at 

 limit of hinge plate, limit of anterior margin submedial, limit 

 of posterior margin supramedial, postero-ventral margin rela- 

 tively flattened, very slightly sinuous particularly in larger 

 specimens; hinge plate relatively short, not extending beyond 

 inner margin of adductor muscles, 5 anterior teeth and 6 

 posterior teeth in specimen 2.5 mm, ventral part of chevron 

 reduced; ligament amphidetic, large elongate internal part, 

 with small external extensions either side of umbo; adductor 

 muscle scars distinct, particularly anterior, line of attachment 

 of pallial sinus also visible. Hydroids may be present, dor- 

 sally, close to umbo and on the posterior and postero-ventral 

 margins. 

 Prodissoconch length: 166-208 |xm. Maximum recorded 



Fig. 51 Yoldiella frigida. Lateral views of left valves showing detail 

 of the ligament and the hinge-plates; a, a specimen from Stor 

 Fjord Spitzbergen (USNM No. 219726); b, a specimen from Sta. 

 105 North America Basin. (Scale = 1.0 mm). 



shell length: 3.98 mm, (Sta. 207), however, specimens from 

 Greenland donated by Ockelmann measure 6.1 mm. 



Comparing height/length, width/length and post-umbonal 

 length/total length ratios of specimens from the North Ameri- 

 can Basin (Sta. 105 & 207) and West European Basin (Incal 

 DS03), it was found that in the latter case the average for 

 each of the three parameters is slightly less than those from 

 the two North American Stations and whose ranges for the 

 most part overlap (Figs. 52 & 53). Specimens from Sta. 105 

 appear to be somewhat wider than specimens from Sta. 207. 



It is highly likely that many records of Y. frigida from the 

 North Atlantic relate to other species. This is a view also 

 expressed by Waren (1989). For example, Y. frigida (USNM 

 No. 193343) figured by Verrill & Bush (1898) and here (Fig. 

 50), closely resembles Yoldiella nana again confirming the 

 observations of Waren (1989). (Y. nana has a straighter 

 dorsal margin, a slightly more inflated umbo and fewer hinge 

 teeth). Further comparing USNM No. 193343 with high arctic 

 specimens of Y. frigida the former has a larger umbo, and 

 thicker hinge. In addition, we believe that many shallow 

 water specimens of Yoldiella inconspicua from the North 

 American Basin have been misidentified as Y. frigida. Subtle 

 differences seen here, as in many other deep-sea protobranch 

 bivalves, are particularly difficult to disentangle (Fig. 53) and 

 in the case of Yoldiella this particularly applies to species 

 found at shallower slope depths. 



Internal morphology (Fig. 54). Mantle structures include 

 a well-developed anterior sense organ and long slender 

 combined siphons with a single lumen. A siphonal tentacle is 

 present originating, but not without exception, on the left 

 side of the inner limit of the siphon embayment. The adduc- 

 tor muscles are unequal in size the anterior being the some- 

 what larger and oval in outline whereas the posterior muscle 

 is more circular in outline. The gill axes attach ventrally to the 

 siphon and with the exception of the ventral margin, the 

 latter must be assumed to be largely exhalent in origin. There 

 is a well-developed feeding aperture ventral to the siphon. 

 The gills are well-developed, the number of filaments range 

 from 12-22 according to the size of the specimen. The palps 

 are large and extend over approximately half the length of the 

 body. There are between 11 and 20 relatively broad palp 

 ridges and the palp proboscides are also well-developed. The 



