58 



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



Fig. 89 Yoldiella biguttata. Lateral views 

 of shells from the right and left sides and a 

 right valve in inner lateral and dorsal view 

 to show variation in shape and hinge-plate 

 details. Specimens from Sta. 301 and Sta. 

 299, Guyana Basin. (Scale = 1.0 mm). 



configuration to shell in lateral view; hinge plate moderately 

 shallow, short, reaching no further than inner limit of adduc- 

 tor muscles, teeth few, anterior and posterior series either 

 equal or with one additional tooth in anterior series (5/6 in 

 largest specimen); ligament amphidetic, very large in relation 

 to size of shell. 



Shell measurements (mm) & ratios are as follows:- 



Length 



Height 



Width 



H/L 



W/L 



PL/TL 



1.64 



1.24 



0.89 



0.75 



0.54 



0.39 



1.62 



1.12 



0.72 



0.69 



0.44 



0.45 



1.50 



1.07 



0.68 



0.72 



0.45 



0.46 



1.50 



1.08 



0.73 



0.72 



0.49 



0.39 



1.37 



1.02 



0.70 



0.74 



0.51 



0.47 



1.19 



0.81 



0.48 



0.68 



0.40 



0.39 



0.87 



0.62 



0.33 



0.71 



0.38 



0.40 



Prodissoconch length: 198 p.m. Maximum recorded shell 

 length: 1.64 mm. 



Internal morphology (Fig. 90). The combined siphon is 

 thin-walled and with a single lumen. In that the gill axis joins 

 the ventral edge of the siphon the inhalent component can be 

 assumed to be largely absent. The siphon is not open ven- 

 trally. A single fine, elongate mantle tentacle originates on 

 the left ventral inner limit of the moderately deep siphonal 

 embayment. An area of secretory cells is present at the base 

 of the siphon. Ventral to the siphon is a well-developed 

 feeding aperture. The posterior adductor muscle is round in 

 cross-section while the anterior is slightly larger and 

 'crescent'-shaped. The visceral and cerebral ganglia are well- 

 developed and joined by a stout commissure. The cerebral 

 ganglia are slightly the larger. The gills have a relatively small 

 number of plates (maximum number recorded 9) most of 

 which are carried posterior to the foot. Labial palp ridges 

 number 14-18, depending on the size of the specimen, and 

 are moderately large. The palp proboscides are also moder- 

 ately large. The foot papillae contain considerable numbers 



Fig. 90 Yoldiella biguttata. Lateral views from the right and left 

 sides of the internal morphology of a specimen from Sta. 299, 

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

 see Fig. 34. 



