﻿NEPHRONAIAS 253 



with two somewhat compressed, but solid, pseudocardinals 

 and two small laterals ; right valve with one pseudocardinal 

 and one lateral ; nacre reddish or greenish, iridescent behind, 

 much thickened in front. The male shell is decidedly rhom- 

 boid, being straight on the base, bluntly pointed at the poste- 

 rior basal part, and obliquely subtruncate on the posterior slope. 

 The female shell is considerably swollen just behind the central 

 base and has the posterior point raised above the base line. 

 The female shell is smaller than that of the male. 



Length (male) 42, height 23, diam. 16 mm. 



Length (female) 36, height 22, diam. 14 mm. 



Type locality, Suwanee River, Madison Co., Florida. Also 

 Ochlocknee River, Georgia. 



IJnio zvalkcri 11. H. Wright, Naut., XL 1897. p. 91. 

 Mcdionidus ivalkeri Simpson, Pr. Ac. N. Sci. Phila., 1900, p. 



77, pi. I, fig. 5 ; Syn., 1900, p. 591. 



Quite distinct from the other species of the genus, as it has 

 a high, well-defined posterior ridge and a dark, rayless epider- 

 mis, which, when fresh, is almost cloth-like. 



Genus XEPHRONAIAS Fischer and Crosse, 1893. 



Nephronaias Fischer and Crosse, Miss. Sci., II, 1893, p. 556. 

 Nephronajas Ortmann, Ann. Car. Mus., VIH, 1912, p. 324. 



Shell elliptical, biangulate behind, that of the male showing 

 a tendency to become arcuate with age, the female usually 

 having a i>osterior inflation and never arcuate ; surface con- 

 centrically sculptured ; beaks low. with faint, broken ridges, 

 which show a tendency to fall into two rounded loops ; pseudo- 

 cardinals generally rather compressed, ragged ; laterals ob- 

 liquely ridged ; cavity of the beaks rather deep, dorsal muscle 

 scars distinct, running in a line from the beak cavity downward 

 and forward. Animal having the branchiffi rounded below, 

 outer the larger behind, inner the larger anteriorly, free from 

 the abdominal sac for all or part of their length ; mantle thick- 



