1 08 THE A M ERICA N NA TURA US T. [Vol. XXX V 1 1 . 



Say, and the sexes can generally be recognized by the shape of 

 the shell; and the same may be said of Syviph. costata Raf. 

 irugosa Barnes). 



We have, then, among the Unioninae with differentiated mar- 

 supia, species in which the female shells are decidedly different 

 from the males, others in which they are slightly so, and still 

 others where differences are not recognizable. Those without 

 differentiated marsupia mostly show slight or no such differences, 

 while in Tritogonia, though of another type, it is well marked ; 

 and again there are marked differences between male and female 

 shells among the Alasmidonta, etc. It is evident from these 

 facts that much weight cannot be placed on this feature for 

 purpose of classification. If so, natural groups would have to be 

 divided, and discrepant forms united. 



The Inner Branchice. — The upper edges of the inner branchiae 

 are adherent to the abdominal sac either immediately, or by an 

 interposed soft membrane of varying width. In some genera, 

 or species, even in the same individual a part of the branchiae 

 may be of one, the balance of the other type. Simpson has paid 

 considerable attention to this feature, and it seems that there are 

 no constant differences available for classification. 



Seasons of Propagation. — Since the publication of my obser- 

 vations several years ago, I have been able fully to confirm the 

 facts then stated. The forms with differentiated marsupia, as 

 Lampsilis, Ptychobranchus, etc., bear embryos during the fall and 

 winter, and discharge their young in spring and early summer, 

 rarely a part of them in autumn. The same was found to be 

 true with the Alasmidonta, Symphynota, etc. On the other 

 hand, the Unio (s. str.), Quadrula, Tritogonia, etc. were invari- 

 ably found with the branchiae barren during fall and winter, their 



