92 
PALAEONTOLOGY OF NEW-YORK. 
from Port Colborne, Canada West, which is apparently identical with it. The finest 
and best characterized specimens which I have seen are from the Falls of the 
Ohio; and it occurs in almost equal perfection at Columbus and Sandusky, Ohio. 
Strophodonta perplana. 
PLATES XI & XII. 
Strophomena perplana : Conrad, Jour. Acad. Nat. Sci. Philadelphia, Vol. viii, p.257. 1842. 
Strophomena crenistria : Hall, Geol. Rep. 4th District, p. 171, f. 4. 
For description and references, see the same species under the Hamilton group. 
A comparison of large numbers of specimens of this species, from the 
base of the Lower Helderberg to the Hamilton and Chemung groups 
inclusive, embracing those described under -several different names, has 
satisfied me that they must all be referred to a single species. There 
is, however, a similar species in the Tully limestone, which in the cha¬ 
racter of its surface strife, the pustulose or punctate interior surface of 
the valves, and the form of the muscular impressions, is very distinct 
from this one. 
This species begins its existence, so far as at present known, at the commence¬ 
ment of the Schoharie grit, where it is not infrequent. It occurs as casts of the 
interior; those of the dorsal valve being the more common. It appears in the 
Corniferous limestone, where it attains large dimensions. It was a ventral valve of 
this species imbedded in limestone, having the surface rather better preserved 
than usual, to which I originally gave the name of S. crenistria. 
The figure 22 of Plate xi is a cast of the ventral valve from the Schoharie grit. 
Figures 13 & 14 of Plate xir are a small and large specimen of the shell, with nearly 
even striae, as it occurs in the Corniferous limestone. 
Figure 15 is a cast of the same species, where the muscular impression is very large. 
Geological formation and locality. In the Schoharie grit, at Clarksville and 
Knox, Albany county and at Schoharie, in the limestone at Williamsville and 
Clarence-hollow, Erie county; at Louisville, Kentucky; and in Indiana. 
