496 MOLLUSCA. 
and rounded shape of the fore part, and in the strong convexity of the valves; inside 
it is of a dull rose-colour, like Fischer and Crosse’s fig. 5 (tab. 60). ‘This shell exhibits 
a curious abnormality in the hinge-line, this. apparently having been broken by 
accident and cemented by brownish crumbling matter (see fig. 3 ¢). 
9. Unio ostreatus. 
Unio ostreatus, Morelet, Test. Noviss. i. p. 29 (1849)’; Fisch. & Crosse, Miss. Scient. Mex., Moll. 
ii. p. 573, t. 63. fig. 8, t. 70. fig. 4°. 
Hab. N. Guatemata: Rio Usumacinta (Morelet } 2). 
The. name ostreatus suggests that the somewhat worn shell resembles an oyster. 
One specimen only known. 
10. Unio percompressus, (Tab. XXXII. figg. 1-3.) 
Unio percompressus, v. Mart. Sitzungsb. Ges. nat. Freunde Berlin, 1887, p. 107°. 
Shell very compressed, the diameter equalling only about 3 of the length or height ; outline trigonal or cordate, 
with very flat sides ; fore part very short, descending above obliquely and somewhat concavely, rounded 
in the lower half; hinder part descending in a more or less convex line in the upper third, almost 
perpendicular in the lower third of its height; ventral margin feebly rounded, with a small sinus, 
followed by a blunt projection at its hinder end. There are, however, remarkable variations in the 
external outline, the upper hinder margin descending in some specimens almost perpendicularly, which 
gives a triangular form to the shell, and the small sinus of the ventral margin is sometimes wanting. 
Sculpture with coarse concentric wrinkles, which are somewhat granulated in the upper half of the shell, 
but usually much worn. The inside in some specimens dull purple, in others with scattered dull orange 
spots, as if stained by oil, in others again pure white. Anterior tooth strong, but not very broad, 
descending almost perpendicularly and as far downwards as the posterior (lateral) tooth. 
a. Long. 98; alt. vert. 76, ale 66, diam. 28 millim. Vertices in 7 long. (Figg. 3, 3a, 6.) 
b. ” 94 ; ” 92, ” 64, ” 284 39 9 4 ” (Figg. 1, 1 a.) 
€. » «99D; ” 73, 5, 67, 4, 24 ” ” 2 ” 
d. ” 73 3 ” 68, 9 60, 99 22 ” ” Z ” (Figg. 2, 2 a.) 
Hab. N. Guatemata: Rio de las Salinas (StoJ/ !). 
Rather near U. ostreatus, Morel., with which, perhaps, when a greater number of 
specimens of different ages are known, it may have to be united, Morelet’s unique 
example representing a younger form. The four specimens of U. percompressus before 
me differ very remarkably one from the other, as will be seen by the figures and the 
measurements; but they agree 7nter se, and differ from Morelet’s U. ostreatus, in the 
following points:—(1) The shell is more compressed, the diameter being about a third 
of the length (in U. ostreatus nearly 3); (2) the anterior tooth is directed almost per- 
pendicularly downward (in U. ostreatus much more obliquely). Moreover, three of the 
specimens have the posterior dorsal margin much more convex, which gives the shell 
a somewhat cordiform, Venus-like outline; whereas one only (4, figg. 1, 1a) has the 
posterior margin nearly straight and the outline, in consequence, triangular, as in 
U. ostreatus. The smallest example (d, figg. 2, 2a), probably not quite full-grown, 
the outside not being worn, comes near Morelet’s shell in size and in the comparative 
