xlviii SYNOPSIS OF GENERA. 



rounded or sub-truncate. Posterior end long, vertically or obliquely truncate, 

 or rounded at tbe 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 processes just beneath the beaks, and in one speci- 

 men a slender oblique fold extends backward from beneatli 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 ti/pa, pi. Ixxix, figs. 26-28, 35-37. 

 PaUzanatina solenoides, pi. Ixxix, figs. 38, 39. 



Prorhynchus, n. gen. [Type, Palaeanatina quadrata, Hall. J 



Shell inequivalve, the left valve the larger and more gibbous ; inequilateral, sub- 

 quadrsite 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- 



