OF DENISON UNIVERSITY. 
8i 
tending forward, terminating in a rounded projection which continues 
the curvature of the shell and thus produces a sinuous outline for the 
front edge of the shell. Surface of the valve rounded into the mod- 
erately concave sinus. Beak less strongly incurved than that of the 
dorsal valve. Cardinal area incurved and directed backward, less, 
however, than that of the other valve. Foramen triangular, wider 
than high; hinge teeth moderately prominent and trigonal; muscu- 
lar cavity oblong, little more than one third the length of the shell, 
lateral margins parallel, well defined by the dental ridges. On either 
side of this cavity are a number of short stri^, which are arranged in 
longitudinal lines following about the direction of the plications. 
Surface of each valve with rounded, radiating plications, from 24 
to 36 in number, of which four to six occupy the mesial sinus, and five 
to seven (in one specimen ten or eleven) the mesial fold. In the 
sinus two plications begin at the beak, two additional ones are imme- 
diately added, and later one or two more at one third or one half the 
length of the shell from the beak. On the mesial fold three plica- 
tions originate at the beak, to which two more are added at one fourth 
the length of the shell from the beak ; later two more appear and in 
one specimen in hand ten or eleven plications are more or less distinct- 
ly shown. The plications in the sinus and on the fold branch in all 
specimens as described above; the lateral ones, 10-15 in number, are 
almost always simple. Lines of growth not shown in the speci- 
mens found. Well preserved specimens under the microscope 
show numerous minute granules, arranged in regular rows across the 
plications. 
Length of the specimen figured, 24 mm.; breadth, 28 mm.; hinge 
line, 21 mm.; convexity, 18 mm.; breadth of largest specimen, 37 
mm. Comparing this description with that given by Meek, in Pal. 
Ohio, Vol. I., it will be found to be about the same as that of var. 
lynx. (The name of the form is intended to suggest the fact that the 
odd and even number of mesial plications are found on valves oppo- 
site to those on which they occur in Lower Silurian forms, as though 
the shell had been tur 7 ied about . ) 
Locality and positmi. Throughout the Clinton Group. Found en- 
tire in the Beavertown marl, generally in fragments in the rest of the 
group. Soldiers’ Home, Centreville, Huffman’s Quarries. 
It may be interesting to notice in this connection that all the forms 
of Orthis biforata from the upper Niagara formations of Ohio, which I 
