PRODUCTELLJE OF THE CHEMUNG GROUP. 
167 
ble in specimens from the skaly 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 extremity, 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 
Lepton a membranacea of Phillips, and which has been recognized as Productus 
( Strophalosia ) murchisonianus by De Koninck. I am quite unable, however, to find 
corresponding characters, and am therefore constrained to adhere to the specific 
designation applied by me in 185*7. 
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, hut in 
many of them Hie 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 two of the figures. Fig¬ 
ure 9 is a ventral valve, which is more rotund and. shorter on the hinge-line than 
usual. 
Geological formation 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. 
