PRODUCTELLiE OF THE CHEMUNG GROUP. 167 



ble in specimens from the shaly beds than in those from the arenaceous 

 portions of the group). In the midst of these are the coarser pustules 

 indicating the spine-bases of the exterior surface. The cardinal process 

 is short and strong, bilobed at the extremitjj with each division less 

 deeply lobed. The process is supported at the base by an oblique ridge, 

 which is often not visibly separated from, the hinge-margin, but in rare 

 examples has shown obscure evidence of teeth-sockets. There is a slender 

 median septum, which is not always preserved, on the interior surface 

 of the dorsal valve. 



This species is subject to considerable variation in form, being often much 

 extended on the hinge, and nearly once and a half as wide as long, while in other 

 examples the length is nearly equal to the breadth : those with the length and 

 breadth nearly equal, are the most gibbous in form. Not unfrequently the 

 shell appears to have been unequally developed on the two sides- of the apex. 

 The species is distinguished from its associates by its wide moderately convex 

 form, concentric striae and very slender spines, the bases of which are usually 

 but little extended on the surface. The interior is more finely papillose than the 

 other species. 



I believe this to be the species which Mr. Vanuxem regarded as identical with 

 Leptana membranacea of Phillips, and which has been recognized as Productus 

 {Strophalosia) murchisonianus by Dk Koninck. I am quite unable, however, to find 

 corresponding characters, and am therefore constrained to adhere to the specific 

 designation applied by me in 1857. 



Figures 1-9 illustrate the prevailing forms of this species. The dorsal valves all show 

 the interior surface, while the ventral valves partially preserve the exterior, but in 

 many of them the shell is more or less removed by maceration and solution. The 

 spines on the surface are rarely preserved, but in some specimens the row of long 

 spines near the hinge-margin is seen in part, as illustrated in twx) of the figures. Fig- 

 ure 9 is a ventral valve, which is more rotund and shorter on the hinge-line than 

 usual. , 



Geological formalion and localities. This species is of common occurrence in 

 the green arenaceous shales of the Chemung group at Philipsburgh and Rockville 

 in Allegany county ; along the line of the Corning and Blossburgh railway ; 

 and at Covington, Pennsylvania. 



