270 



EDWARD S. MORSE. 



abrupt terminations of the mantle converging centrally. Fig. 8 

 shows the relation of these various parts in front of the anterior 

 adductor. 



As the animal becomes enfeebled the mantle (or, what would 

 be a more proper name, the ventral membrane) ruptures, ex- 

 posing the dark-colored gills below. There is a median suture 

 in this ventral membrane and in one rupture the suture became 

 dislocated, showing there was a strain upon it. In another case 

 a small rupture appeared on each side of the median suture. 

 That this membrane limits the expansion of the valves is shown 

 by cutting the membrane, when the valves immediately open 



FIG. 9. FIG. 10. 



FIG. 9. Various attitudes of foot, and ruptures in ventral membrane. 

 FIG. 10. Dorsal view of hinge margin. 



wider in much the same way as when the adductors are severed 

 in other lamellibranchs. Fig. 9 represents the various ruptures, 

 and at the same time illustrates different attitudes of the foot. 

 In the adult the pedal opening extends back nearly half way to 

 the center of the body. In the young it extends a little over a 

 third back. Viewing the shell from above, the exposed ligament 

 presents some curious features which I have tried to represent 

 in Fig. 10. In the vicinity of the umbones a narrow elongated 

 wedge-shaped substance, black in color, is seen, behind which a 

 brown oval ligament appears, split at its posterior end in which 

 is wedged a white substance. From the junction of the two 

 valves anteriorly a narrow wedge-shaped substance narrowing 

 posteriorly is peculiar in being a polished white. In the young 



