﻿NT' PIT RON. MAS 265, 



After carefully comparing von Martens' description and fig- 

 ures of his Unio i^uatemalanus with numerous specimens from 

 various localities I am satisfied that it is this species. Lea's 

 shells (which he has labeled Unio roivellii) from Nicaragua, 

 agree very well with von Martens' description, and they cer- 

 tainly are Lea's goascoranensis. 



Nephronaias dysonii (Lea). 



Shell solid, elliptical, scarcely inflated, strongly and regular- 

 ly sulcate, subbiangular behind, with one or two somewhat 

 angular posterior ridges, the lower the stronger ; beaks slightly 

 prominent ; ligament light brown ; epidermis straw-colored or 

 tawny with one or two darker rays on the posterior slope along 

 as many slight, radial impressions ; left valve with two sub- 

 compressed, ragged pseudbcardinals, with often a third smaller 

 one between them, and two curved laterals, the upper the 

 smaller ; right valve with two pseudocardinals, the upper much 

 compressed, and one lateral ; muscle scars impressed ; dorsal 

 scars on the under side of the hinge plate and the posterior 

 base of the hinder pseudocardinal ; beak cavities not deep ; 

 nacre white, rather silvery. 



Male shell slightly and evenly curved on the base, the pos- 

 terior end a little elevated. Female shell with a moderate post- 

 basal inflation, the posterior end of the shell higher than in 

 the male. 



Length 50, height 32, diam. 17 mm. 



Honduras ; Costa Rica. 

 Unio dysonii Lea, Pr. Ac. N. Sci. Phila., XL 1859, p. 152; 



Jl. Ac. N. Sci. Phila., IV, i860, p. 252. pi. xxxix, fig. 132; 



Obs., Vn, i860, p. 70, pi. XXXIX, big. 132. 

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

 Nephronaias dysonii Simpson, Syn., 1900, p. 594. 



The type is not in the Lea collection, but belonged to Cum- 

 ing, and is apparently a young female shell. There are several 

 shells in the National Museum collection from San Carlos, 

 Costa Rica, which agree so closely with Lea's description and 



