60 



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



ture, ventral margin smoothly curved, posterior margin 

 somewhat more convex than anterior and very slightly 

 extended, anterior and posterior margins broad, rounded; 

 hinge plate elongate, moderately wide except below umbo 

 where narrow, moderately strong, acute taxodont teeth, 7 in 

 anterior and 8 in posterior series in specimen 2.3 mm; 

 ligament internal, amphidetic, slightly elongate, barely 

 extends below hinge plate. 



Prodissoconch length: 172 u.m. Maximum recorded shell 

 length: 2.9 mm. 



Internal morphology (Fig. 94). The inhalent and exhal- 

 ent siphons are combined, the inhalent is open along its 

 ventral margin. The siphonal tentacle is either to the right or 

 left. There is a feeding aperture and anterior to it a heavily 

 ciliated region of the inner mantle fold. There is a well- 

 developed anterior sense organ. The posterior adductor 

 muscle is small and oval, the anterior adductor, also oval, is 

 approximately twice the size of the posterior. The labial palps 

 are moderately large with up to 19 palp ridges and each has a 

 long thin palp proboscis. The gill which is small, dorsal in 

 position in preserved specimens, has up to 14 plates. The 

 visceral and cerebral ganglia are typically 'club'-shaped, the 

 visceral being the smaller. The pedal ganglia are moderately 

 large, elongate and situated high in the foot. The foot is long 

 and thin with deep papillae fringing the sole. A large byssus 

 gland is present in the heel. 



The stomach and style sac are small and the latter does not 

 penetrate far into the foot. The hind gut forms double loops 

 to the left and right of the body, recurring anterior to the 

 stomach, thus taking a similar course to Yoldiella subcircu- 

 laris and Yoldiella biguttata. A considerable amount of fine 

 material was present in the digestive diverticula of the left 

 side. The kidneys although moderately well-developed do not 



50- 

 40- 



80- 

 70- 



60- 

 50- 



W\L 



H\L 



PL\TL 



\ I • ' ' 



Length(mm) 



Fig. 94 Yoldiella ovata. Lateral views from the right and left side 

 of the internal morphology of a specimen from Sta. 300, Guyana 

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

 34. 



penetrate anteriorly into the visceral mass to any great 

 extent. The sexes are separate. One sectioned female (2.7 

 mm) contained approximately 50 ova with a maximum diam- 

 eter of 80 u,m. 



With increasing length, the height/length, width/length and 

 particularly the posterior umbonal length/total length ratios 

 gradually increase (Fig. 93). 



Yoldiella ovata is closely related to Y. biguttata, but can be 

 distinguished by its more ovate outline and not being flat- 

 tened at the postero-ventral shell margin. 



Yoldiella insculpta (Jeffreys 1879) 



Fig. 93 Yoldiella ovata. Variation in the ratios of height H/L, 

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

 length of subsample from Sta. 300, Guyana Basin. 



Type locality. West of Ireland, H.M.S. Porcupine, 1869, 

 Sta. 16, 54°19'N, 11°50'W, 816 fms. 



Type specimen. Holotype: not designated. Lectotype: (here 



