PALAEONTOLOGY OF NEW-YORK. 
9 © 
STROPHODONTA OP THE HAMILTON GROUP. 
As already indicated, several species of Strophodonta pass from the 
Schoharie grit and Corniferous limestone to the Hamilton group. The 
principal among them are the S. demissa, S. perplam, 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. 
Strophodonta concava. 
PLATE XYI. 
Strophomena ( Strophodonta ) concava : Hall in Tenth Report on the State Cabinet, p. 140. 1857. 
Also S. concava : Idem, p. 115. 1857. 
Shell 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. 
