166 [Senate 



Cyrtia hamiltonensis (n.s.)- 



Pal.N.Y. Vol.iv. 



Shell more or less obliquely triangular pyramidal ; hinge equal- 

 ling the greatest breadth, and obtusely angular at the extremi- 

 ties : dorsal valve depressed, nearly flat ; beak scarcely elevated 

 above the hinge-line ; mesial fold small, bounded on each side 

 by deeper and wider grooves than those between the plications, 

 with sometimes a faint narrow longitudinal depresssion in the 

 middle : ventral valve very convex, most prominent near the 

 beak, which is very variable in elevation, and either straight or 

 a little arched from the hinge, sometimes twisted on one side ; 

 sinus distinct, rounded or angular ; area variable, triangular, 

 generally high, often wider than high, arcuate or plane, finely 

 striate in both ways, the vertical striae scarcely visible ; foramen 

 very narrow, usually perforate above by an oval or narrow ovate 

 aperture, and has at its base a small transverse arcuate slit. 

 Surface ornamented by six to eight simple rounded plications 

 on each side of the mesial fold and sinus, and marked by very 

 fine concentric lines of growth. Under a good lens minute gra- 

 nules may be seen on all parts of the exterior except the area 

 and deltidium : interior minutely punctate. 

 This species may be distinguished from the C. dalmani of the Lower 

 Helderberg limestones, by never showing the strongly imbricating lamellae 

 of growth which mark that species. It is, however, more nearly related to 

 Cyrtia [Spirifer) heteroclitus, VoN BuCH ; and until we know the limits 

 of the variation in that species, it will be very difficult to point out cha- 

 ^cters by which our shell can be distinguished from it. 

 Ideological position and locality. In the shales of the Hamilton group : 

 \s of Seneca lake, Moscow, York, Darien, and Eighteen-mile creek. 



Ctktia hamiltonensis. 



