592 UNLO 
Fischer and Crosse have made two varieties of this in the 
Mission Scientifique, semipustulata with pustules over half the 
surface and white nacre and prestricta, which is shorter, more 
inflated with tubercles near the beaks. The examination of a 
number of shells shows so many cross characters that I hardly 
think these varieties need to stand. The species is more in- 
flated and not nearly so high as semigranosus. | have seen a 
large shell that was absolutely destitute of sculpture save some 
slight traces of tuberculation near the beaks. 
UNIo MoRiNI Morelet. 
Shell very inequilateral, long subrhomboid, compressed, 
solid, with a distinct double posterior ridge, which ends in a 
rather wide biangulation behind; beaks rather full, their sculp- 
ture not observed; surface covered with decided concentric 
ridges, which are covered with small granulations or fine nod- 
ules, which are arranged in radiating rows, bifurcating on the 
posterior ridge; epidermis brown; left valve with two pseudo- 
cardinals, the anterior one bifid, the posterior small and oblique, 
with two laterals, the lower larger; right valve with one bifid 
pseudocardinal and one lateral; anterior muscle scars deep and 
rough; posterior scars shallow; nacre white, shaded blue or 
pale rose. 
Length 75, height 47, diam. 28 mm. 
Rio Usumacinta, Guatamala. 
Unio morini Morevet, Test. Nov., II, 1851, p. 24.—FISCHER 
and Crosse. Miss. Sci.j EL, 480459. 576; ple xa. 2 eee 
fig. 4SIMPSON, Syn., 1900, p. 702. 
This seems to differ from U. crocodilarum in having the 
beaks placed much farther forward. It is more compressed 
and wider behind than that species or U. plexus, and the de- 
cided double posterior ridge is a good character, if constant. 
UNIo SEMIGRANOSUS von dem Busch. 
Shell subtriangular, subcompressed, rather solid with a low, 
scarcely double posterior ridge; the region of the beaks promi- 
nent, but the beaks themselves low and compressed, their sculp- 
