SPIRIFERiE OF THE HAMILTON GROUP. * 
shore of Canandaigua 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- 
Spirifera medialis, var. eatoni. 
PLATE XXXVIII. 
Spirifer eatoni : Hall, Tenth Eeport on State Cabinet, p. 157. 
Shell robust, semicircular or sub elliptical, extremely ventricose; binge¬ 
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 
rounded. 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 or 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 sufficient reason for retaining this variety as a species, and 
perhaps a large number of individuals will show an insensible gradation from the 
typical forms of 8. 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. 
