626 UNIO 
Museum by von Ihering. So closely does this resemble a 
large Unio buckleyi that I know of no characters which con- 
stantly differ between the two. It is a little wider and more 
broadly biangulate behind than most specimens of buckleyi; the 
epidermis is a little different from that of that species and the 
nacre becomes thin behind a little more suddenly, but occa- 
sional specimens of buckleyi show the peculiarities of form, 
nacre and epidermis that liebmanni does. I cannot doubt that 
it is a Mexican species and it 1s quite probable that a good 
series showing difterent stages of growth would furnish good 
differential characters. 
Unto opacatus Crosse and Fischer. 
Shell subrhomboid, solid, inflated, with a high, pronounced, 
curved posterior ridge, with full high beaks, whose sculpture 
has not been examined; lunule well marked, the epidermal 
matter extending through under the beaks; there is a sort of 
rounded anterior ridge and in the region of it there are some- 
times curved, radial, elevated lines; anterior end rounded; 
dorsal line curved from the beaks; base straight or decidedly 
incurved, ending in a beak behind; epidermis dark brown or 
blackish, somewhat silky, slightly rayed in the young shell; 
left valve with two ragged pseudocardinals, which are partly 
joined together and two short, curved laterals; right valve 
with two pseudocardinals, the upper generally smaller but 
sometimes equalling the lower in size, with one lateral and 
often a vestigial second; beak cavities rather deep; muscle 
scars impressed, the anterior ones very rough; nacre whitish- 
salmon or purplish-tinted, dull. 
Length 49, height 32, diam. 25 mm. 
Length 58, height 36, diam. 29 mm. 
Mexico, various localities. 
Unio opacatus Crosse and Fiscuer, JI. de Conch., XLI, 1893, 
p. 295.—Fiscurr and Crossr, Miss. Sci., 11, 1894, p. 592, pl. 
LXVI, figs. I, 1@.—SIMPSON, Syn., 1900, p. 712. 
A very solid shell, shaped much like U. plerus Conrad, but 
entirely destitute of nodules. It has a slightly developed an- 
