QUADRULA gol 
Section PAcHyNAIAsS Crosse and Fischer, 1893. 
Pachynaias Crosse and iscHer, Miss. Sci. Mex., Moll., 1893, 
p. 550. 
Shell elongate-triangular, inflated, truncated above and 
rounded below in front, straight on the base, with a high, 
sharp posterior ridge; the post-base slightly biangulate; beaks 
full, the sculpture consisting apparently of faint ridges, which 
run parallel with the growth lines; whole surface distinctly 
concentrically ridged ; epidermis without rays; pseudocardinals 
solid, stumpy, somewhat radiate, roughened; beak cavities 
rather deep; muscle scars deep, smooth; nacre bluish-white, 
thicker in front. 
Animal unknown. 
Type, Unio spheniopsis Morelet. 
QUADRULA SPHENIOPSIS ( Morelet). 
Shell subtriangular, subequilateral, solid, inflated, with a 
high, sharp posterior ridge and 2 low fainter one not far above 
it, with a high umbonal region though the beaks are not very 
prominent; beak sculpture appearing to consist of concentric, 
irregular ridges much like the strong, rather sharp, irregular 
sculpture that covers the shell throughout ; epidermis greenish- 
yellow or yellowish-green, showing a few faint rays behind; 
left valve with two short, remote laterals, the lower higher; 
hinge plate between the two sets of teeth somewhat narrowed 
and rounded; right valve with one strong pseudocardinal and 
a vestigial one above and one lateral; beak cavities deep, 
scarcely compressed; muscle scars distinct, the anterior ones 
deep ; pallial line deep; nacre whitish or bluish-white, thicker 
in front. 
Length 60, height 47, diam. 35 mm. 
Rio Usumacinta, Guatemala; State of Tabasco, Mexico. 
Unio spheniopsis Morrier, Test. Nov., I, 1849, p. 29.—Fiscu- 
ER and Crosser, Miss. Sci., IT, 1894, p. 583, pl. Lxt, figs. 2, 
2a, 2b. 
Ouadrula spheniopsis Stupson, Syn., 1900, p. 794. 
The specimen figured by Fischer and Crosse in the Mission 
Scientifique is not quite adult. The measurements given above 
