594 UNIO 
pustules, which fade out at the border of the shell; epidermis 
dull olive ; nacre rose purple. 
Length 98, height 64 mm. 
State of Chiapas, Mexico. 
Unio corium Rrrve, Conch. Icon., XVI, 1864, pl. x, fig. 39.— 
Simpson, Syn., 1900, p. 702.—von Martens, Biol. Cent. 
Amer., Moll., 1900, p. 495, pl. xxx, figs. 4, 4a, 40. 
I am strongly disposed to regard this as merely an old 
arcuate Unio semigranosus. I have seen a shell; which in 
form stands about midway between Reeve’s figure corium and 
that of von dem Busch’s semigranosus, which I at first re- 
ferred to this species, and afterwards to that of von dem Busch. 
The shell figured by Reeve has fuller, higher beaks than semi- 
granosus. The group containing this and related species is a 
very puzzling one and [ cannot be certain what species should 
stand in it from the limited amount of material | have seen. 
Unio pistincrus Crosse and Fischer. 
“Shell inequilateral, transversely oval, slightly inflated, sub- 
rostrate posteriorly, thick, heavy, covered with an olive-brown 
epidermis with concentric and strong striz, regularly and finely 
reticulate towards the beaks, radiately plicate posteriorly ; an- 
terior end subangulate; posterior subtruncated ; area obscurely 
marked and subangulated; ventral margin curved, slightly 
sinuous; dorsal margin curved in both directions; beaks 
scarcely projecting, somewhat eroded. Nacre of a beautiful 
coppery-brown, tinged with purple. Right valve with two 
short, unequal cardinals much grooved and ragged, (the lower 
much larger and thicker than the upper), with a small, oblique, 
grooved denticle behind them ; lateral tooth oblong and oblique. 
left valve with two cardinals, the anterior thick and larger, 
the posterior shorter, granular, grooved; lateral teeth two, 
lamelliform, elongated, unequal, the lower more prominent. 
Cicatrices of the anterior adductors deep, rugose, irregularly 
oval; those of the posterior adductors superficial, larger and 
subcircular. Ligament quite thick, brownish. 
Length 79, height 50, diam. 33 mm.” (Crosse and Fischer). 
