48 PALEOZOIC FISHES OF NORTH AMERICA. 
they form the segment—from one-third to one-half—of a circle, and are 
from three to four inches in length across the straight side. They were 
apparently four in number, so set as to form pairs of shears. All the speci- 
mens I have are considerably worn, the anterior half of the upper margin 
being beveled off to form a straight, acute, cutting edge. 
In general form and structure these teeth correspond closely with those 
I have described under the name of Rhynchodus frangens, but are smaller, 
narrower, smoother, and much less thick and massive. They are also at 
once distinguishable by their cutting edges. 
On Pl. XXVIII, Fig 1, is represented a tooth of Rhynchodus secans, 
showing a much-worn cutting edge. Fig. 1* represents a section of the 
same. Figs. 2 and 3 are a pair of these teeth placed in their natural posi- 
tions. All these figures are of natural size. Figs 2 and 3 are taken from 
a group of four found together, and forming apparently the dentition of a 
single fish. 
Formation and locality: Corniferous limestone; Sandusky and Dela- 
ware, Ohio. 
RHYNCHODUS FRANGENS, Newb. 
Plate X XIX, Figs. 2, 2%, 3. 
Teeth semi-lunar in form, one side slightly concave, the opposite mar- 
gin strongly convex and regularly rounded; type specimen four inches and 
eight lines in length, depth at the crown of the arch two inches and nine 
lines; sides flattened; greatest thickness, six lines; concave side showing 
a triturating surface on its anterior half and rising upward into a beak-like 
point; lateral surfaces smooth and polished; the lower and rounded portion 
of the tooth formed by thin walls of bone inclosing a deep cavity; crown 
portion thick and strong. 
The above description is based upon a tooth that seems to exhibit an 
unusual perfection of preservation, as there are several in the collection 
before me which apparently represent the crown portion—the massive an- 
terior angle with its beak-like point and triturating surface—while the broad 
and smooth expansion of the sides formed by the thin and shelly portion is 
altogether wanting. It is, however, possible that these are corresponding 
teeth from opposite jaws, of which one form exhibits a crown portion with 
