CHONETES OP THE SCHOHARIE GRIT. H9 



The description of Slrophomena gibbosa, given by Mr. Conrad, is as follows : 

 " Subtrigonal ; inferior valve with back and umbo very prominent and the sides 

 " compressed ; surface with numerous equal slightly undulated strife which 

 " bifurcate on the umbo ; hinge-extremities prominent and angulated. Locality : 

 " Helderberg, in Onondaga limestone." 



The striaB in C. hcmispherica are not undulated, and they rarely bifurcate on 

 the umbo. 



Geological formation and locality. In the Schoharie grit, in the Helderberg 

 mountains and at Schoharie, and rarely in the Corniferous limestone. 



Chonetes arcnata. 



PLATE XX. 

 Chonetet arcuata : Hall, Tenth Report on the Stat*Cabinet, p. 110. 1857. 



Shell semielliptical or approaching to semicircular ; the cardinal extre- 

 mities often extended and auriculate. 

 Ventral valve arcuate, extremely gibbous or ventricose, with usually a 

 shallow undefined longitudinal sinus extending from the umbo to below 

 the middle or near the front of the shell, often constricted near the 

 cardinal extremities : umbo more or less gibbous, and raised in a 

 gentle elevation above the hinge-line, with the beak incurved. Hinge- 

 line, in casts, apparently crenulate ; and on the exterior margin are 

 ten or twelve tubular spines, directed obliquely outwards. 

 Dorsal valve profoundly concave, following nearly the convexity of the 

 opposite valve, and having the centre a little elevated, corresponding 

 to the mesial depression. 

 Surface marked by fine even rounded striae, which increase both by 

 bifurcation and intercalation, crossed by extremely fine concentric 

 striae, with sometimes stronger subimbricating lines of growth. 

 The surface of the cast in the ventral valve is marked by closely 

 disposed oblong pits or pores, from the papillose inner surface of the 

 shell. There is a constricted line extending from the apex, gently rece- 

 ding from, the hinge-margin and curving inwards at some distance from 

 the cardinal extremities, and thence to the front of the shell, leaving the 

 portion outside of this a little more elevated. 



