1881.] NATURAL SCIENCES OF PHILADELPHIA. 421 



resembles the P. Gerardii of Nyst, found in the Coralline Crag 

 of England, but that it differs among other characters in being 

 broader than high, and in having the auricles more developed. I 

 have compared Conrad's specimens with Wood's figure, and find 

 the statement concerning the ears to be correct ; the relative 

 dimensions of the shell are, however, not constant. 



Pliocene. 



I have adopted Tuomey's determination of the South Carolina post- 

 Eocene deposits, there being as yet not sufficient evidence to prove that 

 they are of Miocene age, as insisted upon by Conrad. 



P. affinis Tuomey and Holmes. S. Car. 



Plioc. Foss., p. 26. 



I have seen no specimens of this species. 



P. comparilis Tuomey and Holmes. S. Car. 



Plioc. Foss., p. 29. 



P. eboreus Conr. S. Car. 



Mioc. Foss., p. 48. 

 Tuomey and Holmes, Plioc. Foss., p. 28. 



P. hemicyclus (Rav. ?) Tuomey and Holmes. S. Car. 



Plioc. Foss., p. 25 (name quoted from Ravenel). 



The opercular valve of this species is of the exact outline, and 

 very much the appearance of the similar valve of P. excavafus 

 Sowerby (:= P. Sinensis?), but is considerably less concave. It 

 is of the shape and concavity of P. Jacobaeus, but with more 

 numerous ribs. The right valve is less convex than in P. excava- 

 tus, and wanting on its ribs the prominent lines found in P. 

 Jacobaeus. 



P. Marylandicus Wagner. N. Car. 



J. A. N. S., viii, p. 51, PI. 1, fig. 2 (very poorly figured). 



Described from the Pliocene of North Carolina, but more 

 probably Miocene. 



This species very closely resembles in form, texture and orna- 

 mentation P. Islandicus Miill., but is less prominently ribbed, 

 especially on the ears (where the ribs are also more numerous). 



P. Mortoni Ravenel (Amussium). S. Car. 



Proc. A. N. S., ii, p. 96. 



This species differs from the P. Japonicus Gmel., in its larger 

 size, thinner texture, and in having the internal ribs arranged in 



