THE NAUTILUS. 79 



To this point, proceeding upward in the scale of development, the 

 shells of males and females are essentially alike. It is true that 

 there is some variation in their forms, but it is equally true that some- 

 what elongated specimens with no inflation at the post-basal part of 

 the shell may be females, while shorter specimens that are full post- 

 basally may be males. I do not think there is any strict dimorphism 

 up to this point. In many cases among these lower forms I have 

 assorted my material before opening it, placing in one lot those I 

 would naturally suppose were males, and in another the presumed 

 females, and on opening the shells and examining the animals I 

 always found I was as likely to be wrong as right. In some groups, 

 notably Nodularia and Lamellidens, all the shells of certain species 

 are inflated at the post-basal region. 



Above this point the shells begin to be regularly dimorphic. 

 They are less regularly so in Obovaria, Medionidus, some forms of 

 Nephronaias and Plagiola ; they are nearly always dimorphic in 

 Lampsilis and Truncilla. The shells of male and female are always 

 so different that the merest tyro could without difficulty separate 

 them. 



Von Ihering believes that the Unio tuber culatus of Barnes is nearly 

 related to the U. forsheyi and U. speciosus of Lea. I am surprised 

 at such an opinion, because, while the shells of the two last-mentioned 

 forms are alike in male and female, those of the former are strictly 

 and remarkably dimorphic, that of the female being more compressed 

 and ending posteriorly in a wide, rounded wing. That of the male 

 is more inflated, is truncate behind, and has no wing at all. In the 

 animal of the female there is a wide, rounded flap of the mantle 

 which fills this peculiar extension of the shell, differing somewhat 

 from that of any other that I know of. 



Now among all these higher forms comprised in the group Hetero- 

 gence there is a radical difference in the marsupia. Wherever I have 

 been able to examine them, they occupy only the posterior portion of 

 the outer gills in the form of distinctly-marked ovisacs. Each ovisac 

 when filled is rounded below. The higher the form ranks, the more 

 markedly is the marsupium swollen and separated from the rest of the 

 gill, and the more distinctly is the female shell sivollen in the post basal 

 region to correspond with it. In some forms of Plagiola and Medioni- 

 dus there is little difference between male and female shells. In such 

 cases the marsupium, though having the characteristic ovisacs, is but 



