32 



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



40 

 30 

 80 



60 

 60 

 50 



W/L 





H/L 



■* 





.->' 



PL/TL 





Length (mm) 



Fig. 42 Yoldiella biscayenesis . 

 Variation in the ratios of height 

 H/L, width W/L, and 

 postero-umbonal length PL/TLL 

 to length against length of the 

 three shape forms illustrated in 

 Fig. 41. Solid circles, angulate 

 specimens from Sta. BG VI 

 DS76; solid triangles, rounded 

 specimens from Sta. BG VI 

 DS76; open squares, specimens 

 from Sta. BG II DS31; West 

 European Basin. 



Shell compressed, ovate, fragile, inequilateral, post- 

 umbonal length 48-62% of total length, sculpture of fine 

 irregular concentric lines, pale yellow periostracum; umbo 

 slightly raised, posteriorly directed; antero-dorsal margin 

 convex curving gradually to broadly rounded anterior mar- 

 gin, ventral margin evenly curved, postero-dorsal margin 

 very slightly sinuous, slopes gradually to distal limit of hinge 

 plate, then more sharply inclined to posterior margin; hinge 

 plate moderately long, moderately well-developed with up to 

 10 well developed teeth on each side of ligament, occasionally 

 one additional tooth in posterior series; ligament amphidetic, 

 large, internal, goblet-shaped, extends ventral to and some- 

 what posterior to proximal limit of posterior hinge plate. 



Prodissoconch length: 187-198 p.m. Maximum recorded 

 shell length: 5.82 mm. 



Form B: This was taken between approximately 

 3000-4100 metres and at about 4000 m may be mixed with 

 Form A. The shell is distinguished from Form A by a more 

 marked concavity in the proximal part of the postero-dorsal 

 margin and by a more rounded posterior margin. Internally 

 the hinge and ligament are similar. Form B on average is 

 somewhat smaller than Form A. 



Maximum recorded shell length: 4.19 mm. 



Form C: Recorded at 2076 and 1891 metre depths. This 

 form is intermediate between Forms A and B. It has an 

 almost straight postero-dorsal margin. 



Maximum recorded shell length: 7.39 mm. 



Internal morphology (Fig. 43). The three forms are 

 anatomically indestinguishable from each other. Exhalent 

 and inhalent siphons are combined. There is a siphonal 

 tentacle that lies either to the left or the right of the siphons. 

 A feeding aperture is present ventral to the siphon embay- 

 ment and there is a well-developed anterior marginal sense 

 organ. 



The adductor muscles have conspicuous 'quick' and 

 'catch' parts. The posterior muscle is approximately half 

 the size of the anterior and oval in outline. The anterior 

 muscle is crescent-shaped. The gills are well-developed 

 with a relatively large number (up to 28) of gill plates 

 alternating on either side of the gill axis. The labial palps 



are relatively large and, depending on the size of the 

 animal, have up to 25 internal ridges. The palp proboscides 

 are long and thin. The foot is large with a large byssal 

 gland. There is a small single papilla posterior to the 

 aperture of the gland and the posterior surface of the foot 

 is well-supplied with secretory cells. As in other species, 

 there is histological evidence of a secretion being released 

 along the whole length of the sole of the foot. Large, 

 round, pedal ganglia are situated dorsal to the byssal gland. 

 The visceral ganglia are 'club'-shaped and relatively slen- 

 der, the cerebral ganglia are slightly more inflated. The 

 oesophagus, stomach and style sac are similar to those 

 described for other Yoldiella species, the hind gut is 

 relatively broad and forms a single loop on the right side of 

 the body. The posterior section of the loop describes a 

 conspicuous and characteristic 'S'-shaped course. There is 

 a typhlosole along the length of the hind gut. The digestive 

 diverticula are similar to those of other species of Yoldiella. 

 The sexes are separate. All individuals more than 3.0 mm 

 in length show some gonadial development. A specimen 

 4.9 mm in length contained 190 ova (maximum diam. 150 

 (im). 



Most closely resembles Y. sinuosa and Y. blanda and 

 distinguished by more angulate postero-dorsal margin and 

 lack of postero-ventral sinuosity. 



Yoldiella lata (Jeffreys 1876) 



Type locality. Valorous Sta. 9, Davis Strait, 14.8.1875, 

 59°10'N, 50°25'W, dredge, 1750 fms. 



Type species. Lectotype: U.S. Natl. Mus., No. 199695 as 

 here designated. 



Specimens from Biogas Sta. DS87 are housed in the 

 Museum National d'Histoire Naturelle, Paris. 



Leda lata Jeffreys 1876, p. 431 (in part). 



As is pointed out under Y. jeffreysi (p. 63) when the 

 'Valorous' material was examined, it was found that two 

 species had been grouped together under the name Leda lata. 

 Furthermore, the original description is such that it is not 



