84 SUBCARBONIFEROUS PERIOD. 



sinus ; beak slightly incurvecl over the hinge-line ; ears small ; surface 

 marked by fine, distinct, uniform striae, about two within the space of a 

 millimeter, increasing in number by occasional bifm'cation, rarely by 

 implantation ; faint concentric wrinkles are observable upon the posterior 

 half, especially near the beak, and fine concentric striae are also to be seen 

 under a lens. A few small erect spines are scattered over the surface of 

 the ventral valve, but they become broken off in the imbedding rock, and 

 their bases do not form a conspicuous surface-feature of the shell. 



Length in a straight line from beak to front margin, twelve millime- 

 ters ; breadth, fourteen millimeters ; convexity, about nine millimeters. 



Meek and Worthen's type-specimens of this species w'ere obtained fi-om 

 the Chester limestone of the Subcarboniferous period at Chester, Illinois, 

 and, so far as I am aware, the species has never been recognized elsewhere 

 except at the locality that furnished examples to these collections. Com- 

 pared with the figures and original description given by those authors, our 

 shell is found to difter in no material characters from theirs ; while compared 

 with other known species of similar size and general aspect, they are found 

 to possess characters that clearly separate them from our species. This 

 species has a genei-al resemblance to P. elegans Norwood and Pratten, also 

 from the Chester limestone, P. arcuatus Hall, from the Burlington lime- 

 stone, both, of the Subcarboniferous group ; and also to some varieties of P. 

 longispinus Sowerby, from the Coal-Measm-e strata. It is most nearly related 

 to a variety of the last-named species which is found in considerable num- 

 bers near Santa Fe, New Mexico. In those shells, there is, however, more 

 or less of a mesial flattening of the ventral valve ; the ears also are more 

 flattened and the hinge-line more extended than in the species under con- 

 sideration. 



Position and locality. — Strata of the Subcarboniferous period; Mountain 

 Spring, old Mormon road, Nevada. 



