624 UNIO 
cavity of the shell, whitish or bluish and iridescent at the 
border. 
Length 71, height 37, diam. 27 mm. 
Mexico.: 
Unio sphenorhynchus Fiscuer and Crosse, Miss. Sci., LI, 
1804, p. 617, pl. LXxvI, figs. 2, 2a—SIMPSON, Syn., 1900, 
pei 7ae: 
This shell is apparently related to Unio mitchelli, but 1s 
longer and more drawn out posteriorly than the type and is 
more inflated than the variety elongatus. It differs from all 
allied forms in being decidedly arcuate, the base line being de- 
cidedly incurved in the shell figured. 
Unio RUBICUNDUS von Martens. 
Shell oblong, subrhomboid, solid, subinflated with a well 
developed posterior ridge, which is scarcely double; beaks 
rather high about two-fifths of the way from the anterior end 
of the shell, their sculpture not noted; surface rather strongly 
concentrically striate or sulcate, and having curved plications 
on the posterior slope: posterior end feebly biangulate; post- 
dorsal margin curved; base very slightly incurved; epidermis 
fuscous ; left valve with two crenulated, strong pseudocardi- 
nals and two strongly curved laterals; muscle scars impressed, 
the anterior ones rough; pallial line remote from the border 
anteriorly ; nacre purple. 
Length 80, height 57, diam. 34 mm. 
North Guatemala; Coban. 
Unio rubicundus von Marrens, Biol. Cent. Am., Moll., 1900, 
D514, pl. Xxxiv, figs: 3,36: 
Only a single valve of this is known, and I am somewhat 
at a loss to say where it groups. I, however, incline to place 
it with U. mitchelli and pigerrimus. Its teeth are a good deal 
like those of old specimens of mitchelli and the surface is sculp- 
tured somewhat after the manner of the two forms I have 
placed here. Its widely removed pallial line is a good distinc- 
tive character, and in its strong sulcations it differs from any 
related form. 
