UNIO 591 
The names carbonarius and pliciferus were published by 
Lea for this species two years before plexus appeared, but were 
unaccompanied by descriptions. ‘The species seems to be a 
very abundant one, but I have not heard of specimens being 
found elsewhere than in the vicinity of Vera Cruz. It is 
smaller, darker and more inflated, than any member of this 
group. 
Unio CROCODILARUM Morelet. 
Shell solid, inflated, irregularly elliptical or ovate, more 
rounded on the dorsal outline than on the base; posterior ridge 
well developed, sometimes slightly double and ending in a 
blunt point or feeble biangulation behind; beaks full and high, 
their sculpture consisting of wrinkled, subpustulous bars; sur- 
face either covered with close irregularly radial, subnodulous 
ridges, which often break into regular pustules or smooth; 
these nodules do not extend to the border of the shell; epider- 
mis greenish in young shells, becoming brownish later on and 
black in fully mature shells; left valve with two radial, ragged 
pseudocardinals, the space between them torn and often aris- 
ing into a low third tooth, with two remote laterals; right 
valve with a single strong pseudocardinal and sometimes a 
small one above, and one somewhat double lateral; beak cavi- 
ties well impressed, their dorsal scars under the pseudocardi- 
nals; muscle scars rather deep, the anterior ones rough; nacre 
purplish-coppery or bronzy, sometimes white, thicker in front. 
Length 90, height 55, diam. 41 mm. 
Rio Usumacinta, Guatemala. 
Unio crocodilarum More.et, Test. Nov., I, 1849, p. 28.— 
Reeve, Conch. Icon., XVI, 1864, pl. x, fig. 37.—F1scHer and 
Grosse, Miss. sScr-/LL 1004. “p.. 577. Di Lx, figs. 3, 4, 53 
LXVII, fig. 3. IPSON, Syn., 1900, p. 702.—VON MARTENS, 
S 
SIN 
3iol. Cent. Amer., Moll., 1900, p. 495, pl. xxx1, figs. 3, 3a, 
b, ¢. 
? Unio rusticus SowrErry. Conch. Icon., XVI, 1868, pl. LxIv, 
fig. 324. 
