OF DENISON UNIVERSITY, 
45 
transversely oblong, variable. Ears and middle of ventral margin 
rarely salient. Hinge line nearly or quite equal to the greatest width 
of valves. Ventral valve evenly arched, most prominent one-fourth 
distance from beak to ventral margin ; lateral portions rounded, not 
reflexed; beak moderately prominent, considerably incurved; 'area of 
medium size, marked with numerous obliquely upright ridges ; fora- 
men rather broad; mesial sinus rather sharply triangular in section, 
but not very deep, bearing a medial and four lateral plications, which 
are also impressed upon the cast, but do not extend to the beak, 
however ; costse of the lateral slopes of the valve triangular in sec- 
tion, moderately large, varying in number between to and 14, usually 
not bifurcating, but occasionally increased by implantation, costae to- 
ward the lateral margins, merging into the area before reaching the 
beak. Dorsal valve rather less convex, greatest prominence being 
nearer the middle than in the ventral valve; beak projecting slightly, 
incurved; mesial fold about in correspondence with the sinus, bearing 
five or six simple or bifurcating cost^. The form of the cast of the 
ventral valve is sufficiently illustrated by our figure, also the fact that 
in this species the hinge area of the ventral valve is ribbed — a point 
hitherto unnoticed. 
Spirifera cam^irata , Morton. 
(Plate 11 , Figs. 22.) 
This is the commonest Spirifer and one of the most characteristic 
fossils of the coal-measures. It is found rarely at Flint Ridge and 
Bald Hill in specimens of rather more than medium size. 
Shell somewhat trihedral, the hinge line being longer than any 
other part, often pointed or mucronate. Ventral valve arched, with a 
prominent incurved beak ; area concave, not very acute toward the 
extremities, with an almost equilateral foramen, which is partly closed 
by a pseudo-deltidiiim ; the mesial sinus is deep, triangular in section, 
continuous from the beak to anterior margin. Dorsal valve less convex; 
mesial fold well defined, high; sides of the valves nearly uniformly 
inclined to the lateral margins, beak small, slightly produced. Both 
valves covered with moderately strong radiating ridges which soon in- 
crease by division, the cetral one remaining strongest so that they are 
grouped in fascicles of from three to five ; striae covering both sinus 
and fold; shell also marked by concentric ridges. 
