80 THE NAUTILUS. 



slightly fuller than the rest of the gill. In the more highly organized 

 species of Lampsi/is, both the shell of the female and the marsupia 

 are decidedly produced behind. In Truncilla, which I regard as the 

 highest manifestation of Unione life, the marsupium is almost abso- 

 lutely separated from the rest of the gill, and when full, assumes the 

 shape of a great kidney, projecting below the rest of the branchiae. 

 The great flap of the mantle of the female is very peculiar, being 

 double or having a strong over-hanging ridge inside. In many of 

 the shells of this genus the area corresponding with and covering the 

 marsupium is greatly swollen, is thin, has a different texture from 

 the remainder, is gaping and distinctly toothed. 



I have never been able to examine a gravid female of the Unio 

 tuberculatus of Barnes, hence I cannot give anything more than a 

 guess as to the character of its marsupium, though from some ma- 

 terial lately seen, in which the ovisacs appeared to have just been 

 emptied, I am inclined to believe that the outer gills are filled 

 throughout with embryos, forming well-marked ovisacs. 



Now these remarkable characters of a distinctly separated marsu- 

 pium occupying only the hinder part of the outer gills, and a corre- 

 sponding swelling of the female shell to receive it, the fact that the 

 more distinct and swollen the marsupium is the more pronounced is 

 the swelling of the shell, may be merely the work of chance ; they 

 may stand for nothing whatever in the way of rank or development 

 among our Uniones, but it does not seem so to me. All the changes 

 of shell and marsupium which I have indicated seem to me to be 

 steps in the development of the family from the lowest, simplest and 

 oldest forms to the highest, most complex and most recent. 



I need not occupy space with a discussion on the validity or proper 

 determination of species. Such questions are after all largely matr 

 ters of personal judgment, and in this branch of the work I have 

 endeavored to do the best I possibly could with the material I have 

 been permitted to examine. 



Dr. von Ihering changes the subfamilies UnioninEe and Diplodon- 

 tinse into families, and divides the former into three sub-families, 

 Unionidse, Quadrulina? and Lampsilinie. He gives no characters for 

 these sub-families, and I am totally at a loss to know on what he 

 would found them. Certainly he cannot establish them on beak 

 sculpture, for in nearly all the species this is more or less concentric, 

 and it seems to me does not offer distinctions sufficiently important 

 to be used as a basis for founding sub-families. 



