UNIO 621 
An abberrent form, which perhaps might as well go in the 
buckleyi group as here. Young shells remind one somewhat 
of some of the specimens of Alasmidonta heterodon, but this 
form is more inflated and a little differently colored. 
Group of Unio pigerrimus. 
Shell rather solid, elliptic rhomboid, somewhat inflated, with 
a low, rounded posterior ridge, sometimes a little arcuate be- 
low, biangulate behind; beaks full, sculpture a few coarse 
ridges, which are curved upward and swollen where they cross 
the posterior ridge; surface concentrically striate and often 
sculptured with curved, subradiating or zigzag corrugations, 
which have a tendency to break into nodules ; epidermis brown - 
ish; hinge rather heavy; pseudocardinals stumpy, granular : 
laterals club-shaped ; muscle scars well marked. 
Animal unknown. 
UNIO PIGERRIMUS Crosse and Fischer. 
Shell inequilateral, subelliptical or subrhomboid, inflated 
quite solid, with full high beaks whose sculpture has not been 
observed ; posterior ridge rounded, slightly biangulate, ending 
in a feeble biangulation behind; the posterior slope obliquely 
subtruncate ; surface covered with rude, curved, subradial pli- 
cations, which are divaricate on the posterior ridge; epidermis 
blackish-olive ; hinge thick ; left valve with two pseudocardinals 
the hinder much higher and two laterals; right valve with two 
pseudocardinals, the upper small and one lateral ; anterior mus- 
cle scars deep, very rough; posterior scars shallow; nacre 
white, tinted with rose. 
Length 59, height 38, diam. 27 mm. 
Mexico. 
Unio pigerrimus Crossk and Fiscuer, Jl. de Conch., XLI, 
1893, p. 293.—FiscHeR and Crossr, Miss. Sci., IT, 1894, p. 
582, pl. Lxv, figs. I, la—Simpson, Syn., 1900, p. 711. 
Unio psoricus var. pigerrimus von Martens, Biol. Cent. 
Amer., Moll., 1900, p. 494. 
