596 UNIO 
beak sculpture consisting of faint, doubly-looped, granulous 
ridges; surface covered, except a narrow peripheral border 
which is wider behind, with strong, irregular, closely set tuber- 
cles, the outer border being smooth; epidermis tawny, brown- 
ish or greenish, clouded; left valve with two very ragged, 
partially united pseudocardinals and two curved laterals; right 
valve with two pseudocardinals, the upper smaller and one 
lateral, sometimes with a feeble second one; beak cavities 
moderately deep, compressed; anterior scars rough; nacre 
coppery-purple, considerably thickened in front; pallial line 
remote from the border, especially in front. 
Length 83, height 63, diam. 43 mm. 
Length 74, height 55, diam. 30 mm. 
Rio Usumacinta, Guatemala. 
Unio psoricus Morrtet, Test. Nov., Pt. 2, 1851, p. 25.—FIscH- 
ER and Cross, Miss. Sci., LI, 1894, p. 572, pl. Lx1, fig. 2.— 
Stmpson, Syn., 1900, p. 703.—voN Martens, Biol. Cent. 
Amer., Moll., 1900, p. 494, pl. xxx1, figs. I, 1a, 2. 
A fine specimen of this in the Lea collection agrees very well 
with the description and figure of Fischer and Crosse. It is 
shorter and more triangular as well as solider than the nearly 
allied U. semigranosus of Lea. The posterior slope of this 
species shows slight radiating granulous ridges. The beak 
cavities of this and some related species are compressed and 
are deeper than they usually are in Unio and though the form 
of the shell is nearest to that of members of this genus it is 
possible that these species should be placed in Quadrula. 
Group of Unio gibbosus. 
Shell solid, compressed or inflated, triangular ovate to elong- 
ateovate, arcuate when old, gibbous above, pointed or slightly 
biangulate behind, with a moderate posterior ridge; beaks low, 
the sculpture consisting of a few very strong ridges, which 
run parallel with the growth lines; epidermis dull and cloth- 
like; pseudocardinals strong, rough; laterals granular or ver- 
tically striated, club-shaped; beak cavities very shallow, one 
or more slight furrows or ridges occur in the cavity of the 
