SPIRIFERiE OF THE HAMILTON GROUP. 
227 
The figures 10 - 16 illustrate the prevailing form and characters of this species. 
Fig. 17 is of the interior of the dorsal valve : the interior of the ventral valve is unknown. 
Fig. 18 is an enlargement of the surface. 
Geological formation and localities .- The original specimens described were 
from the Hamilton group at Covington in Genesee county. It has since been found 
among collections from the shores of Cayuga and Seneca lakes; at Dresden in 
Yates county; on the shore of Canandaigua lake, and at Geneseo and York in 
Livingston county. 
Sphifera medialis. 
PLATE XXXVIII. 
Delthyris medialis : Hall, Geol. Report Fourth District New-York, p. 208, f. 8. 1843. 
Spirifer medialis : Hall, Tenth Report on State Cabinet, p. 164. 
Compare Delthyris audacula, Conbad, Jour. Acad. Nat. Sciences, Philadelphia, Yol.viii, p.262. 
Shell sub triangular, semicircular or subelliptical : valves moderately 
convex in young specimens, becoming ventricose in old shells; hinge¬ 
line usually extended beyond the width of the shell below, sometimes 
mucronate at the extremities : surface plicate. 
Ventral valve usually deeper than the dorsal : beak prominent, more 
or less incurved. Area generally of more than medium height, longi¬ 
tudinally striate and divided in the middle by the deltoid fissure, which 
is twice as high as wide, and reaches to near the apex of the valve. 
Mesial sinus of moderate width, reaching to the apex, rather deep, 
generally rounded at the bottom, but sometimes a little flattened; the 
margins subangular and distinct. 
Dorsal valve moderately convex or gibbous, the greatest convexity 
above the middle : beak small and slightly incurved. Area linear and 
well defined ; the lateral extremities of the valve a little deflected 
along the hinge-line. Mesial fold prominent, rising abruptly at the 
sides, rounded or slightly flattened on the top. 
Surface marked by from twenty to thirty simple plications on each side 
of the mesial fold and sinus. These are of medium size in full-grown 
shells, being either low rounded, flattened, sharply rounded, or sub- 
angular in different individuals ; the principal ones are frequently 
marked along the centre by a fine threadlike groove, a feature most 
