UNIO 589 
identification, while his brief Latin description helps out but 
little. Kuster copies his figure, but adds nothing to our knowl- 
edge of the species. In the Lea collection there is a single, 
rather young shell, which was received from Wheatley which 
is labeled “Unio mexicanus Philippi, Mexico,” and it may be 
Philippi’s species. It is rather more inflated in front than be- 
hind, is not quite so decidedly obliquely truncate behind as the 
figures of mexicanis show it to be, but it has a brown epider- 
mis, and pale violet nacre. It may be a young opacatus. 
Group of Unio semigranosus. 
Shell triangular rhomboid, solid, inflated, with a distinct 
posterior ridge; beaks rather full, their sculpture consisting of 
‘numerous somewhat irregular corrugations, which pass into 
the pustulous sculpture of the shell; surface of the valves gen- 
erally more or less sculptured with chevron-shaped or zigzag 
ridges or corrugations, which often break into pustules, the 
posterior slope bearing curved, radiating plications, which are 
likewise sometimes nodulous; epidermis dark, scarcely rayed ; 
pseudocardinals strong, radial, ragged ; laterals heavy, oblique- 
ly striated; beak cavities not deep, compressed; muscle scars 
well impressed; nacre purple, with bronzy or coppery shades. 
Animal apparently not different from that of other related 
Unios. I have not seen any with the marsupium filled. 
Unio PLEXUS Conrad. 
Shell subrhomboid, inflated, solid, inequilateral, with a fairly 
well developed posterior ridge, which is generally feebly bian- 
gulate ; beaks full and high, their sculpture not observed ; dor- 
sal outline behind the beaks curved; anterior end rounded ; 
base straight to slightly rounded; posterior end usually feebly 
biangulate: surface usually sculptured with radiating, curved 
corrugations on the disk; these are divaricate on the posterior 
ridge and they often become somewhat nodulous. Sometimes 
the shell is destitute of sculpture except on the posterior slope ; 
epidermis black or dark reddish-brown, shining when worn or 
