CHONETES OF THE SCHOHARIE GRIT. 
119 
The description of Strophomena gibiosa, given by Mr. Conrad* is as follows : 
“ Sub trigonal; inferior valve with back and umbo very prominent and the sides 
“ compressed; surface with numerous equal slightly undulated strise which 
11 bifurcate on the umbo; hinge-extremities prominent and angulated .Locality : 
“ Helderberg, in Onondaga limestone.” 
The striae in C. hemispherica 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 arena;ta. 
PLATE XX. 
'Chonetes arcuata .- Hall, Tenth Report on the State Cabinet, p. 116. 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 strise, which increase both by 
bifurcation and intercalation, crossed by extremely fine concentric 
strise, 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. 
