20 



The subspecies has been long confused with Y. lucida s.s. 

 (Waren, 1989) and there is little doubt that records of the 

 latter species from deeper than 1000 m are of Y. obesa incala. 

 In large specimens in particular, elongation and narrowing of 

 the posterior margin together with a slightly upturned distal 

 end of the postero-dorsal margin, resembles Y. lucida. A 

 population from Rockall although similar in outline is rela- 

 tively smaller and less inflated than the Biscay populations. 



Prodissoconch length: 187-208 |xm. Maximum recorded 

 shell length: 4.8 mm. 



Internal morphology. Mantle structures are similar to 

 those in Y. obesa s.s., for example the ventral margin of the 

 inhalent siphon is open ventrally, however the feeding aper- 

 ture is not particularly well-developed. The posterior adduc- 

 tor muscle is oval in shape and half the size of the crescent- 

 shaped anterior muscle. There is a large stomach and style 

 sac, and a single loop of the hind gut on the right side of the 

 body. The byssus gland is moderately small. The palp probos- 

 cides are long and palp ridges number up to 21. The gills are 

 well-developed with up to 17 plates present. 



Points of distinction are as follows:- (1) The umbo in Y. 

 obesa incala is slightly more medial than in Y. obesa s.s. (2) 

 The posterior dorsal section of the hind gut loop takes a 

 slightly more anterior course than in Y. obesa s.s. (3) The 

 anterior adductor is relatively more ventral in position than in 

 Y. obesa s.s. (4) Although the ligament is relatively short 

 and, in many specimens rounded as it is in the North 

 American subspecies, in some specimens the anterior and 

 posterior ends are slightly swollen giving a 'dumb bell' 

 appearance. 



In a specimen 3.03 mm length, 281 ova were counted, with 

 a maximum diameter 119 |xm. In two specimens less mature, 

 240 and 387 ova were present in animals measuring 3.06 mm 

 and 4.23 mm respectively. 



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



Yoldiella similiris (new species) 



Type locality. R.V. Atlantis II, Cruise 60. Sta. 236, 

 Argentine Basin, 11.3.1971, 36°27.0'S, 53°31.0'W-36°28.1'S, 

 53°32.3'W, Epibenthic Trawl, 409-518 m. 



Type specimen. Holotype: BM(NH) 1992028. Paratypes: in 

 collection held by J. A. Allen. 



Material. 



Cruise Sta Depth 

 (m) 



No 



Lat 



Long 



Gear Date 



ARGENTINE BASIN 











Atlantis II 236 409- 

 60 518 

 237 993- 

 1011 



86 

 5 



36°27.0'S 



-36 28.1'S 



36°32.6'S 



53°31.0'W 

 53°32.3'W 

 53°23.0'W 



ES 11.3.71 

 ES 11.3.71 



Restricted to shallow slope depths in the Argentine Basin. 

 Depth range: 497-1011 metres. 



Shell description (Figs. 13, 15-17). Shell small, subovate, 

 moderately inflated, smooth, fine irregular concentric lines, 

 inequilateral; umbos small, anterior of midline, inwardly 

 directed; antero-dorsal margin convex, curves evenly to ante- 

 rior margin, ventral margin convexity increases slightly poste- 

 riorly, posterior margin obliquely subtruncate, postero-dorsal 

 margin approximately straight, gently sloping, posteriorly 

 forming a blunt angle, dorsal margins on either side of umbo 

 form an acute edge; hinge plate well-developed, anterior 

 slightly arched, posterior almost straight, 11 anterior and 12 

 posterior strong angular teeth in individual 3.75 mm long; 

 ligament amphidetic, small secondary external periostracal 

 extensions on either side of umbo, internal primary ligament 

 moderately large, extends below hinge line. 



Prodissoconch length: 198 u.m. Maximum recorded shell 

 length: 3.75 mm. 



The shell and internal morphology of Y. similiris is very 

 similar to that of Y. lucida s.s. The following differences are 

 noted:- The umbo of Y. similiris is slightly more anterior, this 



Fig. 13 Yoldiella similirus. Lateral views of shells from the right 

 side and a hinge plate of a left valve. Specimens from Sta. 236 

 Argentine Basin. (Scale = 1.0 mm). 



Fig. 14 Yoldiella similiris. Lateral view of internal morphology 

 from the right side. (Scale =1.0 mm). For identification of parts 

 see Fig. 5. 



