Welter — Kinderhook Faunal Studies. 33 



straight; posterior margin backwardly truncate or slightly 

 sinuate below, rounded above into the cardinal margin; 

 anterior margin rather sharply rounded below, becoming 

 straightened above and meeting the cardinal margin in a 

 rounded obtuse angle. Hinge-line straight posteriorly, turned 

 downward in front, the straight portion sub-parallel with the 

 basal margin. A prominent rounded umbonal ridge extends 

 from the beak to the posterior extremity of the basal margin 

 and another similar but less conspicuous ridge extends from 

 the beak to the most posterior extension of the shell. The 

 area between these two ridges is depressed, forming a con- 

 spicuous sinus. The whole surface of the shell covered with 

 fine angular concentric costae about -J mm. apart upon the 

 outer position of the shell, but becoming much more closely 

 crowded towards the beak. 



lietnarks. In the original specimens of this species from 

 the Waverly Group of Ohio, the anterior margin is quite 

 regularly rounded, while in the Northview specimens it is 

 rather sharply rounded below and straightened above. In 

 all other respects the agreement is close. P. marshallensis 

 Win. is a closely allied species and may possibly be identical 

 with P. truncata, 



SCHIZODUS AEQUALIS Hall. 



Pl.IlLf.lO. 



Height of shell about 35 mm. in the larger examples, che 

 length a little greater than the height. Basal margin regu- 

 larly curved; the anterior margin broadly rounded below, 

 becoming straighter towards the cardinal line ; posterior 

 margin truncated somewhat vertically. Hinge-line short. 

 Valves regularly convex, the greatest depth about 10 mm., at 

 a point somewhat above the middle point of the shell. Beaks 

 subcentral, elevated above the hinge-line. The umbonal ridge 

 obtusely subangular, extending from the beak to the posterior 

 basal extremity. The posterior cardinal slope concave. 

 Surface marked by fine concentric striae which are more 

 conspicuous towards the margin of the shell. 



Remarks. None of the Northview specimens of this species 

 are entirely complete. So far as they are preserved, how- 



