﻿TRITOGONIA 317 



Unio gabbianiis Lea, Pr. Ac. N. Sci. Phila., Xll, 1868, p. 95 ; 



Jl. Ac, N. vSci. Phila., \I, 1868, p. 295, pi. xliii, % 106; 



Obs., XII, 1869, p. 55, pi. XLiii, fig-. 106. 

 Margaron (Unio) gabbianiis Lea, Syn., 1870, p. 35. 



Lea's type is a female shell, and the type of his Unio gabbi- 

 aniis is a male shell, which I believe belongs to the same 

 species. I have examined a large amount of material belong- 

 ing to this group and of this species and it seems to vary much 

 in the degree of inflation and the length of its shell. 



DIGEN.E. 



Male shell inflated, truncated posteriorly ; female shell com- 

 pressed, expanded into a broad wing behind the post-ridge, the 

 wing being filled with a flap of the mantle. 



Genus TRITOGONIA Agassiz, 1852. 



Tritogonia Agassiz, Arch, fiir Nat., 1852, p. 48. 



Shell solid, elongate rhomboid, having a strong, irregular 

 posterior ridge ; obliquely truncated behind in the male ; in the 

 female this region is somewhat truncated and rounded ; base 

 incurved ; whole surface, except the rounded wing of the 

 females, covered with pustules; beaks rather low, incurved 

 and turned forward over the well-developed lunule, which is 

 elongated and filled with epidermal matter ; beak sculpture 

 strong, consisting of irregular, subparallel ridges, which are 

 curved upward behind, and fine radiating ridges in front of 

 and behind this ; epidermis dark olive : hinge plate rather nar- 

 row ; pseudocardinals strong, ragged ; laterals long and 

 straight, near to the pseudocardinals. 



Animal having the inner gills much larger than the outer, 

 generally free for the most part from the abdominal sac ; palpi 

 enormous, elongated, united to each- other behind and to the 

 mantle a part of their length ; mantle thin, with a thickened, 

 dark, double border, the inner edge often toothed throughout, 

 the base much thickened at the posterior end and folded at the 

 branchial opening : branchial opening large, with numerous 



