PALEONTOLOGY OF NEW-YORK. 



STROPHODONT^ OF THE HAMILTON GROUP. 



As already indicated, several species of Stropiiodonta pass from the 

 Schoharie grit and Corniferous limestone to the Hamilton group. The 

 principal among them are the S. demissa, S. perplana, S. inequistriata and 

 S. nacrea : of these, the two first pass into the Chemung group. It may, 

 perhaps, be questioned also whether the S. hemispherica of the Corniferous 

 limestone may not assume the form to which I have given the name S. 

 concava in the Hamilton Group. 



Those species which begin their existence below the horizon of the 

 Hamilton group, and pass into the Chemung group, have their greatest 

 development in the Hamilton group, in number of individuals, in size, 

 and in development of parts, with perhaps the exception of the extrava- 

 gant forms of S. perplana in the Chemung group. 



Stropiiodonta concaya. 



PLATE XVL 



Strophomcna (StropKodonta) coneata : Hall iq Tenth Report on the State Cabinet, p. 140. 1857. 

 Also S. concava : Idem, p. 115. 1857. 



Shbll large, from two to three and a half inches wide on the hinge-line, 

 concavo-convex or subhemispheric, broadly semielliptical or subcircu- 

 lar, sometimes subtriangular from becoming narrowed in front. The 

 proportions vary from nearly equal length and breadth to a width one- 

 fourth to one-third greater. The hinge extremities are sometimes 

 salient, but often rounded. 



Ventral valve varying from moderately to extremely convex, and be- 

 coming gibbous in the middle, rounded upon the umbo and little elevated 

 above the hinge-line, with beak small and scarcely incurved : in some 

 specimens, the centre of the valve is elevated in a median ridge. 

 Dorsal valve usually almost flat or slightly concave in the upper and 

 central portions, becoming suddenly deflected towards the margin ; in 

 some specimens, regularly concave. 



