. PRODUCTELL^ OF THE HAMILTON GROUP. 163 



arranged undulating concentric strias. 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 diflScult, 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 Hamiiton 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. 



Prodactella exanthemata. 



PLATE XXIII. 

 Producttu exarUKematut : Hall ia 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 



