ORDOVICIAN FAUNAS. 167 
ually into the broadly and uniformly-curved posterior margin; basal 
line most prominent and strongly convex behind the centre, in front 
of which point it ascends rather rapidly, with a much more gentle 
eurve, into the short, small and sharply-rounded anterior end. Um- 
bones full, large and prominent, beaks small and strongly incurved; 
umbonal ridge subangular near the beaks only, inconspicuous in a 
lateral view. Surface marked with concentric lines of growth. These, 
with the exception of a few near the margin, are obscure in the 
material at hand. lLigamental area very narrow. Hinge-plate of 
moderate strength, with three slightly-curved and nearly-horizontal 
cardinal teeth and two or three slender, posterior, lateral teeth in 
each valve. Pallal line and anterior adductor muscle distinct, the 
latter rather small and of obovate or subcircular shape; posterior ad- 
ductor faintly impressed, situated immediately beneath the lateral 
teeth. Internal umbonal sulcus and ridge slightly developed, but 
always distinguishable on good casts of the interior.”—UlIrich. 
The dimensions of a somewhat imperfect specimen are: length, 17 
mm. ; -height, 18 mm.; thickness through both valves, 12 mm. 
vemarks.—This species has not been found abundantly in the Tren- 
ton limestone of New Jersey, but the specimens seem to be identical 
with the more typical representatives of the species from the Missis- 
sippi valley, except in being somewhat smaller. 
CYRTODONTA CANADENSIS Bill. 
IPERS Dog hie” GY, 
1858. Cyrtodonta canadensis Bill., Can. Nat. and Geol., vol. III., p. 
434, figs. 8-10. 
Description.—“Transversely broad-cval; anterior, posterior and 
ventral margins, and also the posterior half of the dorsal margin, 
regularly rounded; a portion of the ventral margin about the centre 
of the width is sometimes nearly straight; dorsal margin elevated, 
somewhat compressed; diagonally and rounded ventricose from the 
umbones towards the posterior ventral angle; beaks short, obtusely 
rounded, incurved; surface nearly smooth or obscurely marked with 
concentric ridges; a few strong, imbricating lamelle of growth near 
the margin of some specimens.”—Billings. 
