PRODUCTELLiE OF THE HAMILTON GROUP. 
fl63 
arranged undulating concentric striae. The dorsal valve, in its upper 
part, is marked by rounded fossets, while on the middle and lower 
part the depressions become, elongate grooves. No spines have been 
observed. 
A careful examination of the hinge-line of the ventral valve has shown 
a narrow area, with foramen and small teeth. In one specimen, preserv¬ 
ing the two valves, there is a barely perceptible separation of the mar¬ 
gins for a short distance along the middle of the hinge-line; while in 
another similar specimen, there is no such separation perceptible. 
This species is from half an inch to an inch in length and diameter. The varia¬ 
tions in its different stages of growth, and the variable number of spines upon 
its surface, render it difficult, with the materials before me, to point out the cha¬ 
racters which separate some of the smaller specimens, on the one hand from P. 
navicella, and the other from some forms of P. shumardiana. 
Geological formation and localities. This fossil occurs in the Hamilton group, 
at Tinker’s falls and Delphi falls in Onondaga county; at Bellona in Yates coun¬ 
ty ; on the shore of Canandaigua lake ; at Moscow in Livingston county; and 
near Hamilton in Madison county, New-York. 
Productella exanthemata. 
PLATE XXIII. 
Productus exanthematus : Hall in Tenth Report on the State Cabinet, p. 174. 1857. 
The original of this species is a moderately concave dorsal valve, the 
interior of which is closely studded with pustuliform elevations, indi¬ 
cating fossets on the exterior of the shell. Near the umbo these eleva¬ 
tions are nearly round, becoming oval below, and sometimes forming 
short oblique wrinkles. The cardinal process is distinctly bilobed; and 
there are obscure indications of teeth-sockets. 
The characters are dissimilar to those of the interior of valves of well-marked 
specimens of P. shumardiana , 'though many of the dorsal valves of that species 
show numerous short ridges on the interior surface^; but I have not been able to 
find a gradation to this character, Among the collections from the Corniferous 
limestone; there are several other dorsal valves with a similar cardinal process, 
and round pustuliform elevations in the upper part of the shell, while they 
