SPIRIFEKiE OF THE HAMILTON GROUP. 221 



Spirifera sculptilis. 



PLATE XXXV. 

 Dtlthyris tatlptilit : Hall, Geol. Report of Fourth District New-York, p. 202. 1843. 



Shell gibbous ; valves subequally convex, semielliptical or subtriangular : 

 hinge-line longer than the width of the shell, and prolonged into 

 mucronate extensions; length about half the width on the hinge-line. 

 Surface coarsely plicated.  . 



Ventral valve regularly convex, arcuate ; beak arcuate over a sublinear 

 area of moderate height, extending to the limits of the cardinal line ; 

 mesial sinus strongly defined, subangular. 



Dorsal valve regularly convex, the greatest convexity in the middle 

 and regularly arcuate from beak to base; mesial fold abruptly and , 

 strongly elevated, with the summit flattened or grooved ; beak in- 

 curved : area very narrow. 



Surface strongly marked by three, four or five abruptly elevated angular 

 plications on each side of the mesial fold and sinus, leaving a some- 

 what wide corrugated space at the cardinal angles. The plications 

 bordering the sinus are stronger, more elevated, and continuing dis- 

 tinct quite to the apex. The shell is concentrically marked by strong 

 imbricating lamellose striae, which are abruptly bent backwards and 

 much elevated in crossing the plications, giving them a subnodose 

 character. In the bottom of the sinus, these lamellose striae have often 

 a distinct retral bend, with a slight elevation indicating an incipient 

 plication which corresponds with the depression in the mesial fold. 



ThJs species is readily recognized by its few strong plications, and the wide 

 space at the cardinal extremities marked only by the concentric strife. Compara- 

 tively few specimens have been found, and these are partially exfoliated. 



In the figures illustrating this species, the dorsal valve is a cast. 



Geological formations and localities. This shell, in its characteristic form, 

 occurs in a calcareous layer in the Hamilton shales, at Eighteen-mile creek. It is 

 found at Ludlowville in Cayuga county ; on the east and west shores of Seneca 

 lake, and at York in Livingston county. 



