PRODUCTELL^ OF THE CHEMUNG GROUP. 171 



A careful observation of the hinge-line of the ventral valve discloses 

 an extremely narrow flat space, which may be regarded as an area with 

 a narrow open aperture ; but so obscure are these features, that, unless 

 examined with great care, they may be readily overlooked. 



The cast of the ventral valve shows elongate semioval imprints of the 

 occlusor muscles on the umbo, and wide spreading strongly striate diva- 

 ricator muscular impressions. 



In the interior of the dorsal valve we find a slender mesial septum 

 which terminates in a bilobate cardinal process, the shell being some- 

 what thickened at the sides, and probably having minute dental sockets ; 

 but these have not been distinguished. The entire interior surface of both 

 valves is minutely papillose, and these points become more elongate on 

 the middle of the shell, and finally towards the front are extended in 

 extremely slender striae, which are likewise impressed upon the cast. 

 The figures 19-23 of Plate xxiv illustrate the usual form and condition of this spe- 

 cies ; one specimen preserving two of the spines, and showing the bases of others on 

 the surface of the valve. 



This species occurs in the same beds ^vith P. hirsute and P. hoydii, and is rea- 

 dily distinguished from either of these by its greater gibbosity and fewer spines ; 

 while the casts may be recognized by the extremely fine papillae, and their exten- 

 sion in capillary striaj. In general characters, it resembles P. subalata of the Ham- 

 ilton group at Rock island, Illinois ; but it is much more convex, and the ears less 

 extended than the usual form of the specimens from the western locality. The 

 specimens before me are not in a condition for satisfactory comparisons of the 

 two forms in all their parts ; but I would suggest that such comparisons be insti, 

 tuted, with a view to determine the relations of the species. 



Geological formation and localities. In the arenaceous shales of the Chemung 

 group at Philipsburgh in Allegany county, and on the line of the Corning and 

 Blossburgh railroad. 



