. Bulletin of the Natural History Museum Zoology. 58 ALLEN, 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 = 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 la
. Bulletin of the Natural History Museum Zoology. 58 ALLEN, 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 = 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 Prodissoconch length: 198 Maximum recorded shell length: 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
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