SPIKIFER^fl OF THE HAMILTON GROUP. 335 



Sjpirifer congesta (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. Sjnrif&r dintoni (10th Eep. 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 xxxvii). 



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 

 dimcniiions 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 (aS. 

 congesta, ut cit.), the umbo of tlie 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 foi-ms 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 gi-oup 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. 



f 



SpIBIFEEA ORAKUUnKA 



[Paleontology IV.] 29 



