76 



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



60^ 



40 



W\L 



BU- 











H\L 



• 



• • • 

 • • •• 



• 



60^ 









50-i 







40- 



PL\TL 



..... 



• 



■ 





i 

 2 



Length (mm) 



1 



3 



Fig. 121 Yoldiella fabula. Variation in ratios of height H/L. width 

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

 of specimens taken by the Biogas Expeditions as listed in the 

 table of material. 



ovate, inequilateral, sculpture, fine concentric lines and 

 growth rings, no lunule, no escutcheon; umbo large, poste- 

 rior to midline, directed to posterior; antero-dorsal margin 

 merges with anterior margin in smooth curve, limit of ante- 

 rior margin close to midline, ventral margin long, shallow 

 curve, limit of posterior margin ventral to midline, proximal 

 postero-dorsal margin slopes more steeply than antero-dorsal 

 margin and results in narrowing of post umbonal shell in 

 lateral view; hinge plate moderately strong, except ventral to 

 umbo where it is very narrow, hinge teeth strong, somewhat 

 elongate, 1 or 2 more teeth in anterior series; ligament 

 amphidetic, largely ventral to hinge margin; no chondro- 

 phore. 



Prodissoconch length: 165 u.m. Maximum recorded shell 

 length: 4.56 mm. 



Internal morphology (Fig. 125). The combined siphon 

 has a single lumen which is open ventrally, as in Y. fabula. 

 The siphonal embayment is deep, a fine tentacle is present on 

 the right side. The adductor muscles are particularly large but 

 approximately equal in size, the anterior being fractionally 

 the larger. The anterior sense organ is not particularly 

 well-developed. The gill is elongate and attenuated with up to 

 27 plates. The palps are large each with up to 13 internal 

 ridges. The foot is also large and with papillate margins 

 anteriorly directed. The stomach is moderately large and the 

 hind gut forms a single loop to the right side of the body. The 

 hind gut has a wide diameter and a single typhlosole is 

 present along its entire length. 



This species has clear affinities with species of Portlandia. 

 Nevertheless, it is a fragile shell without any trace of lunule or 

 escutcheon. On the basis of only three specimens, the shell 

 morphology seems most similar to Y. fabula. 



Fig. 122 Yoldiella fabula. Lateral view from the right side of the 

 internal morphology of a specimen from Sta. 85, North America 

 Basin. (Scale = 1.0 mm). For identification of parts see Fig. 34. 



Fig. 123 Yoldiella fabula. External views of intact stomach an style 

 sac in a, left; b, right and c, antero-frontal views. Dissected from 

 a specimen from Sta. 85, North America Basin. (Scale = 1.0 

 mm). 



Genus Portlandia Morch 1857 



Type. By selection (Soot-Ryen, 1984: Opinion 769 ICZN) 

 Nucula artica J.E. Grey, 1824. 



Shell moderately small, slightly inflated, moderately frag- 

 ile, oblong, posteriorly angular, more or less truncate, sub- 

 rostrate, not gaping, escutcheon present, usually defined by 

 weak or occasionally moderate carina, lunule may be present; 

 usually glossy, smooth, postero-ventral margin may be sinu- 

 ous, proximal postero-dorsal margin almost straight or 

 slightly concave; umbo prominent, anterior, chondrophore 

 variously developed; ligament amphidetic, largely internal 

 with small part external, hind gut single loop to right of body. 



Portlandia lenticula (Moller 1842) 



Type locality. Greenland. 



Type specimen. Lectotype BM(NH) 1843.7.3.31, right-hand 



