. Bulletin of the Natural History Museum Zoology. . 4-, 3- E E o> 4) .C (0 2- 1- i r 1—r 5 T T "I—r 9 No. of teeth Fig. 103 Yoldiella Jeffrey si. The relationship of the number of hinge-teeth to shell length of a subsample from Sta. 316, West European Basin. The left and right limits of each bar indicate the number of anterior and posterior hinge-teeth respectively. anterior of the body, thereafter describing an 'S'-shaped bend before continuing as an anticlockwise loop which partially overlies the first. Thereafter it runs parallel to and immedi- Fig. 102 Yoldiella jeffreysi. a, later
. Bulletin of the Natural History Museum Zoology. . 4-, 3- E E o> 4) .C (0 2- 1- i r 1—r 5 T T "I—r 9 No. of teeth Fig. 103 Yoldiella Jeffrey si. The relationship of the number of hinge-teeth to shell length of a subsample from Sta. 316, West European Basin. The left and right limits of each bar indicate the number of anterior and posterior hinge-teeth respectively. anterior of the body, thereafter describing an 'S'-shaped bend before continuing as an anticlockwise loop which partially overlies the first. Thereafter it runs parallel to and immedi- Fig. 102 Yoldiella jeffreysi. a, lateral view of a right and left valve of a specimen from the Jeffreys collection labelled Leda lata (USNM No. 199695, Valorous Expedition, Sta. 9). b, a dorsal and lateral view from the right side of a specimen from Sta. INCAL DS 06, West European Basin. (Scale = mm). ately below the dorsal margin to the anus (see Fig. 107). A typhlosole is present along its entire length. The configura- tion of the hind gut on the right side of the body, despite | being somewhat variable is characteristic of this species (Fig. 108). It can be very simply visualized as a doubled length of rope making one and a half turns (Fig. 107). In this species, more than any other we have examined, there is a consider- able variation in the course of the hind gut which can easily be resolved by reference to the rope analogy (Fig. 107). It would appear that this manner of accommodation of a long hind gut is particularly susceptible to distortion during development, possibly because of displacement due to the large size of the stomach and the anterior arrangement of the digestive diver- ticula or possibly because the loop tends to impede its own development in this particular configuration (Fig. 105). The nervous system is well-developed with moderately' large 'club'-shaped cerebral ganglia, slightly smaller elongate visceral ganglia and large, oval, pedal ganglia. Dorsal to the latter are large, round,
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