1 6 CONK LIN. [Vol. X 1 1 1 . 



the females, and all are motile. In C. fornicata the males may 

 become almost as large as the females, in which case they 

 become immovably fixed to one spot, and cannot, therefore, 

 perform the sexual function unless they are attached near to 

 or upon a female. In C. convexa and in C. adunca all the 

 males are smaller than the females, and are motile. I have 

 carefully taken the volume of a number of alcoholic specimens, 

 and find that the following ratios exist between the males and 

 females of the different species : in C. plana the males are 

 about one-sixteenth the size of the females ; in C. adunca, 

 one-eighth ; in C. convexa, one-fifth ; in C. fornicata, three- 

 quarters. The small males are able to move about more or 

 less freely ; if they are detached they readily find a new foot- 

 hold, and their shells are rarely distorted to fit irregular sur- 

 faces, as is the case with the females. There is, then, a marked 

 sexual dimorphism in these mollusks, the mature females being 

 generally much larger than the males ; the females are seden- 

 tary, the males locomotive, and at the breeding season, or 

 perhaps once for all, the females are visited and fertilized by 

 these motile males. In all mature females, the seminal recep- 

 tacle, which is a convoluted tubule communicating with the 

 oviduct, is at all times filled with mature spermatozoa. These 

 spermatozoa are attached to the walls of the receptacle by their 

 apices, while their tails project into the lumen exactly as they 

 do in the seminiferous tubules of the male. I believe that the 

 spermatozoa receive nutriment from the walls of the seminal 

 receptacle, and that they can live in this position indefinitely. 

 Since there are myriads of spermatozoa in the receptacle, and 

 furthermore, since none are wasted, so far as I have been able 

 to observe, it might well be that copulation occurs only once 

 in a lifetime. 



In C. plana the shell of the male has a characteristic shape, 

 being more nearly round than that of the female, and having 

 a rather sharply pointed apex. This shape is so characteristic 

 that it is generally easy to distinguish a male from an immature 

 female. I have observed a good many cases in which the 

 older part of the shell had the male characters while the newer 

 part was like that of the female. In such animals the penis is 



