1 IO 



A LI ST. [Vol. XXXVII. 



>r the main divisions, they might 

 ) Haplodontes. 



The former again contains two main subdivisions, — one in 

 which the marsupia are differentiated, the other in which they 

 are not so. Among the first. Ptychobranchus represents a group 

 by itself, equivalent to the balance, owing to the exceptional 

 formation of its branchiae and ovisacs, and features of the shell. 

 A group represented by C. irrorata Lea, reJUxa Raf. and 

 dromus Lea seems a natural one. although there are differences 

 in the number and size of the ovisacs, which might be regarded 

 as sufficient for generic distinction. And so it is with the bal- 

 ance, of which Lampsilis is the typical genus. The differences 

 between male and female shells in the several groups of Trun- 

 cilla are more considerable than between some of the Truncilla 

 and Lampsilis s. str., and so it is in regard to some other features. 



Simpson ranges under the Lampsilis group also Cristaria 

 Schum. and Pilsbryoconcha Simpson, the hinge teeth of which 

 ait cetettne or almost wanting, and the soft parts unknown, as 

 arc als«. the embryos. With a knowledge of the latter, their 

 pos.t,on will be more fully ascertained. In regard to Pseudo- 

 spatha we refer to what has been said above 



Fn the second subdivision of the Uniones. Tritogonia is 

 (l|stln - lll>hcd b y its shell. Unio. IMeurobema and Ouadrula, 



bivalves, and comprising various plastic and variable forms ^are 

 so closely related and almost inseparably connected, that even the 

 highest authority is in doubt under which of these ^enera certain 



t^h Vh^b be ranged- In regard t0 the branchije ' we refer 



It may be in place here to mention that these mussels quite 



silis, etc., do. And this seems to be in else Ln^ion 'with the 

 tact that their posterior mantle openings, especially the branchial, 

 are muc ess fringed than those of Lampsilis, the fringes having 



the function of 

 terials with the 



as been seen by 



