UNIO 593 
ture consisting of irregular, subnodulous ridges; surface more 
or less covered with close set, small tubercles, which sometimes 
form nodulous or corrugated bars, especially on the posterior 
slope, the border of the shell smooth; epidermis yellowish- 
brown; base line straight or slightly curved; post-dorsal line 
almost evenly curved from the beaks to the base, but sometimes 
showing a faint, biangulation below; left valve with two rag- 
ged, subcompressed pseudocardinals and two remote laterals, 
the hinge line between the two sets of teeth. very narrow ; 
right valve with two pseudocardinals, the upper smaller and 
one, sometimes slightly double, lateral ; beak cavities compress- 
ed, moderately deep; muscle scars shallow, the anterior ones 
rough; nacre coppery, thicker in front; pallial line rather dis- 
tant from the border. 
Length 110, height 65, diam. 32 mm. 
Mexico, Vera Cruz to Tampico. 
Unio semigranosus VON DEM Buscu, (in Philippi), Abbild. und 
Besch., I, 1845, p. 109, pl. 1, figs. 1-3—Hantey, Biv. Shells, 
1856, p. 381. pl. xx, fig. 33—lKuster, Conch. Cab. Unio, 
1861, p. 252, pl. L.Xxxv, fig. 1—Simpson, Syn., 1900, p. 702. 
—Vvon Martens, Biol. Cent. Amer., Moll., 1900, 493, pl. 
May, figs. I-4. 
Margaron (Unio) semigranosus Lea, Syn., 1852, p. 20; 1870, 
P- 34- 
Ouadrula semigranosa Piispry, Pr. Ac. Nat. Sci. Phila., 1909, 
P- 532: 
Unio carbonarius var. semigranosus Pavey, Conch. Sam., ITI, 
TOOO; Pp. 147. 
The Lea collection contains a fine specimen of this shell 
presented by von dem Busch, and I have given its measure- 
ments above. It is a little straighter on the base, and a trifle 
more elongated than the specimen figured in Philippi, but it 
is a more adult shell. 
Unio cortum Reeve. 
Shell long rhomboid, subcompressed, with a low, somewhat 
double curved, posterior ridge, with rather high full beaks, 
whose sculpture is not known; surface covered with close-set 
