﻿3lH TRlTdGONIA 



crowded papilh'e ; anal openin;^ smooth or with only fine dentic- 

 ulations ; superanal opening Ions;, closed helow ; in the female 

 there is a thickened flap of the mantle, which fills the circular 

 posterior expansion of the shell, and which has a smaller flap 

 inside ; foot and ahdominal sac larj^e. the latter winged in 

 front. Marsnpiam occn]iving all four gills. 



Type, Unio 7'crnicosiis Say. 



The genus Tritogonia is one of the most remarkable in the 

 Unionidcc. In all other genera and groups where the shells 

 are sensibly dimorphous the female shell is swollen and ex- 

 tended at the posterior base in front of the posterior ridge, 

 while the male shell is without this swelling. But in Trito- 

 gonia none of the shells are produced at the posterior base, 

 but that of the female is produced behind the posterior ridge, 

 spreading out in a thin, wide, rounded wing or flap at the 

 extreme hinder end of the shell, while that of the male is 

 oblic|uely truncated below and ends behind in a more or less 

 decided point. In the other dimorphic groujis the female shell 

 has, as a rule, a greater diameter than the male, and is never 

 more compressed. But in this peculiar form the male shell 

 is much more inflated generally than that of the female. 



Tritogonia tubercuuata (Barnes). 



Shell large, elongated, solid, subrhomboid, scarcely inflated, 

 inequilateral, with a decided, curved, elevated ridge, which 

 ends well forward on the jiosterior base, in front of which 

 for a long distance there is a shallow excavation ; beaks low, 

 compressed, sculjjtured with strong, irregular, corrugated 

 ridges that turn up behind ; surface covered as far back as the 

 posterior ridge with irregular, different sized pustules, which 

 sometimes form somewhat chevon-shaped figures ; behind the 

 ridge the sculpture consists of strong, irregular, cur.ved. cor- 

 rugated and often knobbed ridges; epidermis greenish-brown, 

 or brown, dark green in young shells ; left valve with two 

 ragged, triangular jiseudocardinals and two straight laterals; 

 right valve with one large pseudocardinal. a small one in front 

 of it, and vestigial teeth behind it, with one lateral, which in 



