72 IOWA STUDIES IN NATURAL HISTORY 
the West Indies. Perhaps the most striking group at Makuluva 
was the Nudibranchiata, some of which were among the largest we 
had seen. Their colors were gorgeous beyond description; scarlet, 
crimson, white and blue being the most conspicuous. Unfortunate- 
ly, we were unable to preserve the larger ones and the smaller 
ones were but sad relics of their former selves by the time they 
had been immersed in formalin for even a few hours. The colors, 
of course, are totally evanescent. We doubtless missed many good 
things among the mollusks on account of not having with us the 
keen-eyed John B. Henderson, to whom the securing of the fine 
series from Barbados and Antigua was due. We were practically 
confined to forms found on the surface and reefs of the flats, as 
there was no bottom, over which a net could work successfully 
because of the coral heads and rocks. Professor Thomas did the 
greater part of the work on mollusks, although he had but a few 
days at Makuluva. 
Lamellibranchs were proportionally few and belonged to well 
known genera. The most conspicuous, of course, was T'ridacna, 
which has been already referred to, belonging to a family charac- 
terized by the absence of siphons and the presence of a very 
strong byssus with which it anchors itself to the rock. The spec- 
imens on the flats were all small, the deeper water probably being 
a better habitat for the big fellows. 
As I have never seen any detailed description of the soft parts 
of this giant clam the following notes may be useful. The dorsal 
edges of the valves are widely separated back of the umbo to ac- 
commodate the extraordinarily strong byssus connected with the 
muscular, cylindrical foot expanded towards its distal end which 
bears the mass of byssal fibers. The shell has two faint pallial 
sutures corresponding to the double edge of the mantle which fits 
closely the deeply scalloped edges of the strong valves. In some 
specimens the opening for the byssus is unsymmetrical in that the 
right valve is reflected back over the dorsal surface more than the 
left. The muscle sears are hardly evident on the shell; this seems 
strange to one who has noted the power with which the shells 
grip any intruding object. There is a very large posterior ad- 
ductor muscle but no anterior adductor, and the attachment of 
the former to the valves is farther toward the ventral edges than 
is usual in the lamellibranchs. The muscle itself is enormous and 
the surface of attachment to the valves correspondingly large. 
This muscle mass is plainly divided into two parts and the ques- 
