494 MOLLUSCA. 
Notwithstanding the differences in form and locality, Morelet’s species appears to me 
to be nothing more than a young form of U. semigranosus. 1 have before me three 
specimens of the latter, two of them old and thick (figg. 8, 3a, 6, from Tampico, 
and figg. 4, 4a, 6), with strong cardinal teeth, the granulation occupying only about 
half the surface, and the ventral margin being feebly convex; but the lines of growth 
show that when these examples were young the ventral margin was much more 
convex and the whole shell covered with granulations. Another specimen, from 
Tampico, given by A. Gruner to Prof. Dunker (figg. 2, 2a, 6), is somewhat more 
compressed, with the outside of a brighter brown colour, the inside paler, salmon- 
coloured, with more feeble teeth; these peculiarities indicate that it is a younger 
shell, although it has the same length as the smaller one of the normal examples. 
Moreover, in this young specimen the ventral margin is more convex and the 
granulation extends nearer to the ventral margin; it differs only from Fischer and 
Crosse’s figure of U. testudineus in having the summits somewhat more advanced, in 
2 of the whole length, instead of § asin U. testudineus. A very young specimen is 
also figured (figg. 1, 1 a). 
Var. distinctus. 
Unio distinctus, Crosse & Fisch. Journ. de Conch. xli. p. 110 (1893) °; Miss. Scient. Mex., Moll. 
li. p. 580, t. 69. figg. 2, 2a, b”. 
Hab. E. Mexico: Rio Cosamaloapam (confluent of the Rio Alvarado), near Chacal- 
tianguis, State of Vera Cruz (Sallé® 1°). - 
The figure given by Crosse and Fischer 1° is very like that of U. semigranosus, only 
it represents a comparatively lower shell. 
5. Unio psoricus. (Tab. XXXI. figg. 1, 1a, 2.) 
Unio psoricus, Morelet, Test. Noviss. i. p. 25 (1849) '; Fisch. & Crosse, Miss. Scient. Mex., Moll. 
li. p. 572, t. 61. fig. 3. | 
Shell granulated in its upper part, as in U. semigranosus, but more swollen and with the hinder part bent 
downward ; the ventral margin at first moderately convex, then sinuated, and finally projecting, corre- 
sponding to an obtuse ridge descending from the summits. 
Hab. N. Guatemata: Rio Usumacinta (Morelet!*); Rio de las Salinas (Stol/). 
The general outlines vary to a certain extent, being either somewhat triangular- 
rhomboid or oblong, the length of the shell ranging from 14 (fig. 1) to 1% (fig. 2) of the 
height in specimens from the same locality. The inside is either purple or whitish. 
Var. pigerrimus. 
Unio pigerrimus, Crosse & Fisch. Journ. de Conch. xli. p. 293 (1893) *; Miss. Scient. Mex., Moll. 
li. p. 582, t. 65. figg. 1, la’. : 
Hab. Mexico (Sallé 34). 
Rounded, elliptical, the granulosities comparatively large and arranged in arcuated 
lines, as in a specimen of U. cortum in the Berlin Museum. 
