SPIRIFERiE OF THE HAMILTON GROUP. 
£28 
This shell resembles S. sml mm ik} but it is more gibbous and has a larger num¬ 
ber of plications, extending quite to the auriculate cardinal extremities ; the sinus 
is also wider 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 offer characters which are apparently inter¬ 
mediate, having coarser lamellae than the original specimens of Sjpirifera 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-fourths as great. 
Geological formations and localities. In the Hamilton group, at Moscow, York, 
Geneseo, Darien, and Eighteen-mile creek, and less frequently on Seneca and 
Cayuga lakes. 
Spirifera grauulifera. 
PLATES XXXVI & XXXVII. 
Delthyris granulifera : Hall, Geol. Report Fourth District New-York, pp. 206 & 207, f 1. 1843. 
“ congesta : Id. Ib. f. 2. 
Spirifer heteroclitus ; Marcotj’s Explanatory Text, etc. PI. 8, f. 7. 
<« granulifera : Hall, Tenth Report on State Cabinet, p. 163. 
Compare S. clintoni, Id. Ib. p. 157. 
«« S. duplicate,, Conrad, Jour. Ac. Sciences, Philadelphia, Vol. viii, 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 the greatest width of 
the shellj 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 fropt, and 
often having a slight groove along the centre, with sometimes an in¬ 
dication of one or two small faintly-marked plications just within the 
margins. 
