418 proceedings of the academy of [1881. 



Miocene. 

 P. biformis Conr. Va. 



Proc. A. N. S., i, p. 306. Mioc. Foss., p. 73. 



This species appears at first sight to be closely related to the 

 P. Danicus of Chemnitz, from which, however, it can be readily 

 distinguished by the profound notch under the ear of the right 

 valve, the greater concavit}^ of the opercular valve, and the more 

 prominent and irregular lines on the inferior moiety of the ribs of 

 the convex valve. 



P, oerinus Conr. Md. 



A. J. Conchol., V, p. 39. 

 P. comparilis Tuomey and Holmes. Va. ; N. Car. 



Plioc. Foss., p. 29. 



P. eboreus Conr. (in part). 



= Specimen marked by Conrad as P. Yorkensis. 



P. mkropleura (young) H, C. Lea. Ti-ansactions Am. Philosoph. 

 Society, ix, p. 245. 



The specimens which constitute the P. comjmrilis of Tuomey 

 and Holmes were included by Conrad in his P. eboreus, but the 

 two can be readily distinguished from each other in the character 

 of the ribs, which in P. comparilis are considerably more elevated, 

 and much more distinctly marked off from the general surface of 

 the valve. I have retained as the type of Conrad's P. ehoreus the 

 forms agreeing with the figure in tlie " Fossils of the Medial 

 Tertiary of the United States." The P. comjmrilis bears in many 

 respects a close resemblance to P. purpuratus Lam., from the 

 coast of Peru, but it may be easily distinguished by the ribs in 

 the interior faces of the valves passing prominently to the umbonal 

 region, wliereas in P. purpuratus they become indistinct a short 

 distance from the margin, and appear, moreover, much broader. 

 P. comparilis is, again, less ventricose on the umbonal region, but 

 more convex toward the basal margin. The intermediate scaly 

 rib between the principal ones present in P. purpuratus is wanting 

 in P. comparilis. The ears are in P. comparilis less prominently 

 marked by the radiating lines. 



P. decemnarius Conr. Va. 



J. A. N. S., vii, p. 151. Mioc. Foss., p. 49. 

 P. dispalatus Conr. Va. 



Mioc. Foss., 74. 



