SPIRXFER2E OF THE HAMILTON GROUP. 
225 
Sjpirifer congestd (Geol. 4th District) is a specimen of this species having a short 
hinge-line and very obtuse cardinal extremities, a narrow area, incurved beak, and 
a mesial fold simply flattened instead of the usual groove; while the granulose 
character of surface is scarcely perceptible. Sjpirifer dintoni (10th Rep. on State 
Cabinet) was founded upon small specimens of this species occurring in a band of 
limestone, and having the granulose surface entirely removed by exfoliation, while 
the plications and sinus have a. sharper and more angular expression, and the car¬ 
dinal extremities are more extended than usual (See Plate xxxvti). 
The specimens figured on Plate xxxvi illustrate the prevailing forms observed 
in the calcareous shales of the Hamilton group in Western New-York. Smaller 
specimens than figure 1 are extremely rare, while it is not often observed of greater 
dimensions than fig. 10. The elevation of the beak of the ventral valve as shown 
in figures 3 and 7 is the prevailing character; though in specimens like fig. 6 ( 8. 
congesta , ut cit.), the umbo of the dorsal valve is much more elevated, while the 
beak of the ventral valve is less extended. The interior characters of the dorsal 
and ventral valves of well-formed and full-grown individuals are shown in figures 
11 & 12. Some casts of this and allied forms are figured on Plate xxxvm a. 
Geological formation and localities. This species is known in the Hamilton 
group in Schoharie, Otsego and Madison counties, usually in the condition of casts. 
It is coextensive with the Hamilton group in the State of New-York, occurring 
on the shores of Cayuga, Seneca and Canandaigua lakes; at Geneseo, Moscow 
and York in Livingston county; at Pavilion and Darien, Genesee county, and at 
Eighteen-mile creek in Erie county, as well as numerous other places. The variety 
with more angular extremities and mesial fold, which I have designated as S. 
dintoni, is known only in a calcareous band at Eighteen-mile creek. The species 
likewise occurs in Pennsylvania, Maryland and Virginia. 
[ Paleontology IY.] 
29 
