SPIRIFER^ OF THE HAMILTON GEOUP. 223 



This shell resembles S. sciilpHUs; but it is more gibbous and has a larger num- 

 ber of plications, extending quite to the auriculate cardinal extremities ; the sinus 

 is also ^ider and the front more elevated. Usually only the coarse lamellae are 

 conspicuous upon the surface, the finer striae being obscure or covered by adher- 

 ing matter. 



Figures 13 and 14 of Plate xxxv ofier characters which are apparently inter- 

 mediate, having coarser lamella than the original specimens of Spinfera zigzag; 

 and it was from specimens of similar form that I described 8. clio. 



This species varies in dimensions from half an inch to an inch and a quarter in 

 width, with a length two-thirds to three-fourtlis as great. 



Geological formations and localities. In the Hamilton group, at Moscow, York, 

 Gcnesco, Daricn, and Eighteen-mile creek, and less frequently on Seneca and 

 Cayuga lakes. 



Spirifera granulifera. 



PLATES XXXVI & XXXVII. 



DMhyrU granulifera : Hall, Geol. R»port Fourth District New-Tork, pp. 206 & 207, f 1. 1843. 



" congetta : Id. lb. f. 2. 

 Spiri/er heteroclUtu : Mabooc's Explanatory Text, etc. PI. 8, f. 7. 



•< granulifera i IIall, Tenth Report on St&te Cabinet, p. 163. 

 Compare S. c/in<(mt, iD.lb. p. 167. 



«« S. duplicata, Conkad, Jonr. Ac. Sciences, Philadelphia, Vol. Tlii, pa. 261, pi. 14, f. 16. 



•• S. arcta i Hall, Tenth Report on the State Cabinet, p. 161. 



Shell semicircular, semielliptical or subcircular, ventricose, often be- 

 coming gibbous ; binge-line generally equal to tbe greatest width of 

 the shell ; cardinal extremities obtuse, or sometimes rounded, rarely 

 extended : valves subequally convex ; surface plicate. 



Ventral valve less ventricose than the dorsal, most prominent near the 

 umbo, gradually sloping to the cardinal extremities ; beak moderately 

 elevated, somewhat pointed and incurved. Area of moderate height, 

 divided in the centre by a rather large triangular foramen which is 

 about as high as wide. Mesial sinus distinct and subangular in the 

 upper part, becoming a broad deep rounded depression in front, and 

 often having a slight groove along the centre, with sometimes a,n in- 

 dication of one or two small faintly-marked plications just within the 

 margins. 



