xlviii 
SYNOPSIS OF GENERA. 
rounded or sub-truncate. Posterior end long, vertically or obliquely truncate, 
or rounded at the margin. Beaks low, appressed, that of the left valve rising 
above that of the right. Cardinal line straight or gently arcuate. Umbonal 
slope prominent and extending to the post-basal extremity. A broad, shallow 
depression extends from the umbo to the base of the valve, causing a shallow 
sinus in the ventral margin. 
Surface marked by fine concentric striae of growth. 
Ligament partially external; a ligamental groove, just within the dorsal 
margin, extending nearly to the post-cardinal extremity of the shell. Hinge 
furnished with two slender jn’ocesses just beneath the beaks, and in one speci¬ 
men a slender oblique fold extends backward from beneath the apex; no 
lateral teeth have been observed. Muscular impressions indistinct; the anterior 
one small, situated just forward of the beak; the posterior one larger and near 
the cardinal margin of the valve. Pallial line apparently parallel to and near 
the ventral margin of the valve. 
The shells of this genus are more distinctly characterized by the inequality 
of the valves than by any other feature. The known species occur in a coarse 
arenaceous matrix, which renders their condition unfavorable for a critical 
study of the internal or external characters. These forms seem to have no 
near allies among the associated fossils of the same or other formations. 
Examples: Palaanatina typa, pi. lxxix, figs. 26-28, 35-37. 
Palaanatina solenoides, pi. lxxix, figs. 38, 39. 
Prorhynchus, n. gen. [Type, Palaeanatina quadrata, Hall.] 
Shell inequivalve, the left valve the larger and more gibbous; inequilateral, sub- 
quadrate or rhomboidal. Anterior end truncate, angular or nasute at the antero- 
dorsal extremity. Posterior end broad, margin truncate or broadly rounded. 
Beaks low, appressed, the umbo of the right valve somewhat gibbous and rising 
above the hinge-line; beak of left valve scarcely above the cardinal line. 
Cardinal line straight, extending the entire length of the dorsal margin, 
and alate at both ends. Umbonal slope sub-angular, extending to the post- 
