CYRTINA OF THE HAMILTON GROUP. 267 



v.ilves, the spires may be directed towards the cardinal extremities as- 

 in Spirifeea. 



The original specimen described was too imperfect to admit of a fidl description, 

 but the characters indicated have been observed in all the specimens obtained. 



The illustrations, Plate xxvii, figs. 5-10, repi'esent the prevailing features of 

 the species. 



Geological fofmations and localities. This species occurs in the Schoharie grit, 

 in Albany and Schoharie counties; and in the Corniferous limestone, at Williams- 

 ville in Erie county. I have also a specimen from the same rock in Michigan, re- 

 ceived from Dr. Rominoer of Ann-Arbor. Although having so wide a geographical 

 range, this species is a rare fossil. 



Cjrtiiia crassa ( n. 8.). 



PLATE XXVII. 



Shell depressed-pyramidal (semielliptical in a dorsal view) ; length and 

 breadth about as three to four ; hinge-line equal to the greatest width 

 of the shell, with the extremities slightly rounded. 



Ventral valve depressed-semipyramidal, convex, regularly arching from 

 the beak and cardinal area to the front ; sinus broad and rounded in 

 the bottom ; beak extended and slightly incurved over the area, which 

 has a height about equal to half the length of the valve. 



Dorsal valve moderately convex, a little inflected or concave towards the 

 cardinal extremities ; mesial fold broad, moderately elevated, rounded 

 above and strongly defined : area linear. 



Surface marked on each side of the mesial fold and sinus by about four 

 strong, low, rounded plications, which are crossed by fine threadlike 

 concentric striae and a few imbricating folds. Shell-structure punctate. 

 A single specimen only of this species has been seen; but its gredt lateral ex- 

 tension, comparatively low area, and few strong plications, preclude its union with 



any of the previously described species. 



The figures 11 & 12 of Plate xxvn are a dorsal and ventral view of this species. 

 Geological formation and locality. In the Corniferous limestone, near Vienna, 



Ontario county. 



