SPIRIFERJE OF THE HAMILTON GROUP. 339 



shore of Canaudaigua lake, and at Geneseo, Moscow and York in Livingston 

 county; at Covington and Darien in Genesee county, and at Eighteen-mile creek 

 in Erie county. It is extremely rare, if occurring at all, in more western localities- 



Spiiifera medialis, var. eatonl. 



PLATE XXXVni. 

 Spirtfer eatoni : Hau, Tenth Report on State Cabinet, p. 157. 



Shell robust, semicircular or subelliptical, extremely ventricose ; hinge- 

 line equalling or less than the width of the shell below : valves sub- 

 equally convex : surface plicate. 



Ventral valve much elevated in the umbonal region, sloping gradually 

 to the cardinal extremities, which are either obtusely pointed or 

 rounde^r Mesial sinus moderately wide, not deep, flattened on the 

 bottom, angular at the sides, and reaching to the beak. Area high, 

 straight or but little incurved, with a narrow deltoid opening reach- 

 ing to or near the apex of the valve : beak small, pointed, slightly 

 incurved. 



Dorsal valve convex, a little flattened o]> slightly inflected near the 

 cardinal extremities : mesial fold well marked, rising abruptly at the 

 sides and flattened on the top. Area narrow linear. 



Surface marked by from fifteen to eighteen simple rounded plications 

 on each side of the fold and sinus, with sharply defined narrow spaces 

 between, and crossed below the middle by strong imbricating lines of 

 growth. 

 Interior of the shell unknown. 



There is probably no suflBcient reason for retaining this variety as a species, and 

 perhaps a large number, of individuals will show an insensible gradation from the 

 tjTjical forms of S. medialis to the more robust and ventricose forms with short 

 hinge-line and fewer plications. 



Geological formation and localities. In the Hamilton group, on the shore of 

 Seneca lake; at York in Livingston county, and probably at other places. 



