Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/24

 belong to a species otherwise unknown. In general form they are triangular with a broad flat manubrium for articulation or the attachment of muscles. Inner or masticating side arched laterally, concave to the dorsal side of the carapace. Crown lunate, broadest anterior to the middle, and furnished with a row of usually six, more or less, bicuspid denticles. The denticles become more prominent and acute towards the posterior extremity of the crown, where they are also somewhat oblique, while anteriorly they are broad and obtuse. The test forming the crust of the mandible reaches a thickness of more than one mm. over the denticles, but becomes very thin over the expansions of the sides and on the manubrium. For this reason very few of the specimens preserve more than merely the row of denticles.

M. J. Barrande has figured and described several specimens of a similar nature and has referred them to the genus Ceratiocaris from their association with species of this genus in the Silurian of Bohemia. It is therefore probable that these portions of the animal do not furnish sufficient generic character to be of importance.

Formation and localities.—In the shales of the Hamilton group; at Delphi, Pratt's Falls, and the shore of Cayuga lake, New York.

convex, obliquely ovate in outline, widest posterior to the middle, length about one-third greater than the width. Dorsal line straight, oblique, having a length of more than one-half the longest diameter of the valves. Margins thickened, carinated, curving outwards over the anterior end of the valves at nearly right angles to the direction of the hinge, continuing in a broad curve to the lower posterior end, where the margin is abruptly rounded, thence extending in nearly a straight line to the hinge.

Cephalic region triangular in each valve, distinctly limited by a furrow extending from the center of the dorsum, curving outwards and downward to the anterior margin,