SPIRIFERiE OF THE UPPER HELDERBERG GROUP. 
Figure 23, Plate 31, illustrates the ventral valve of an individual of this spe¬ 
cies. From the conditions of preservation, it does not represent the full length of 
the valve in proportion to the width. Mr. Billings has recognized a similar form 
in Canada West, which he compares with S. mucronata. Our specimens are very 
unlike $. mucronata. 
Geological foi'mation and locality. In the Upper Helderberg (Corniferous) 
limestone, near Williamsville, New-York. 
Spirifera segmenta. 
PLATE XXXI. 
Spiri/er se> mentus : Hall in Tenth Report on the State Cabinet, p. 131. 1857. 
“ “ Description of'New Pal. Fossils, p. 91. 1857. 
Shell transverse, semioval; length less than half the width : hinge-line 
equalling the greatest width of the shell, and terminating in salient 
angles. Surface plicate. 
Y entral valve much elevated, subpyramidal, most prominent at the 
beak, which is not incurved : sinus strongly defined, shallow and 
nearly flat in the bottom, with the sides straight, giving a triangular 
form, in which the sides are about once and a half as long as the base. 
Area very large with sharply angular margins, a little inclined for¬ 
ward, and nearly of the same size as the exterior of the valve : the 
fissure is high and large, being nearly of the same dimensions as the 
mesial sinus. 
Dorsal valve depressed convex, and flattened towards the cardinal extre¬ 
mities, larger than the ventral valve, semielliptical in form, with a 
low but sharply defined mesial fold which is barely flattened upon the 
summit. The proportions in height of area, length of dorsal and length 
of ventral valves, is about as five, six and seven. 
Surface marked by twenty or more simple rounded (or subangular) pli¬ 
cations on each side of the mesial fold and sinus, the lateral ones of 
which do not reach the beak, but run out along the margin of tl: ® 
In its perfect condition the shell has been marked by- fine concentric 
striae, traces of which are still preserved, together with stronger imbri¬ 
cating lines of growth. 
