219 
SPlRIFERJS OF THE HAMILTON GROUP. 
Dorsal valve moderately convex; the mesial fold strongly defined, wide 
below and narrow above, flattened and sometimes depressed upon the 
summit; the lateral portions of the shell curving gently to the sides, 
with the cardinal extremities flattened. 
Surface marked by from sixteen to eighteen comparatively wide flat¬ 
tened or low rounded ribs on each side of the fold and sinus; the ‘ 
exterior ribs fine and obscure. In exfoliated specimens, and sometimes 
in others, the ribs are subangular. The entire surface (including the 
mesial fold and sinus, and particularly the intervals between the pli¬ 
cations) marked by fine continuous radiating strise; while faint con¬ 
centric striae are visible on the fold and sinus, as well as obscurely on 
the costae, and more decidedly upon the cardinal extremities; but 
these are subordinate to the radiating striae. 
The casts show the muscular area of the ventral valve to be elongate, 
and margined above by long dental plates which are comparatively strong 
as shown in figure 8. In another specimen, figure 9, there has been an 
elevated median crest in the muscular area. 
The figures 1 and 6 of Plate 35 illustrate the prevailing character. 
In general aspect this species resembles S. medialis ; but it is more gibbous 
and proportionally less in width than the young of that shell, while the radiating 
strife are a distinctive feature. The area also is more arcuate than in any forms of 
8. medialis. 
It is with hesitation that I propose a new species of Spieifera, while I am com¬ 
pelled to unite several that have hitherto been regarded as distinct; but in this 
one the characters seem to be distinguishing, and irreconcilable with any of the 
species heretofore described. 
Geological formations and localities . In the shales of the ^Hamilton group, on 
the east side of Skaneateles lake, Owasco lake, Borodino, and in loose masses of 
the rock near Tally, Apulia arid other places in Onondaga county. It is not at 
present known from more western localities. 
