480 THE AMERICAN NATURALIST. [Vow. XXXII. 
by the lateral wall of the jaw. In all other species the knife- 
edge is shorter than the tritoral area, but in the present form 
the cutting edge is never shorter, and may be as much as one- 
fourth longer than the tritors. In the lower jaw it is the out- 
side and in the upper the inside wall which is thus sharpened 
into a razor edge. As a whole, the jaws are straighter than in 
P. calceolus, and the symphysial region differently formed, as 
is apparent from the figures given herewith, the originals of 
which are preserved in the Museum of Comparative Zoology, 
and were all collected by the writer near North Liberty, Iowa. 
Fic. 35. — Ptyctodus ferox, sp. nov. Left upper dental plate. 4. 
(6) P. ferox, sp. nov. (Figs. 35-40). — Much larger, heavier, 
and rarer than any of the foregoing is the species which we 
will call by this name. Less than a dozen examples have been 
obtained, all told, from the two localities where they occur, 
namely, the State Quarry fish-bed and the Hydraulic Cement 
quarries of Milwaukee, Wis. Those from the latter horizon 
are exceptionally well preserved, and were obtained by Messrs. 
Teller and Monroe. One of the four upper dental plates in 
Mr. Teller’s collection (shown in Fig. 35) was very kindly pre- 
sented by him to the Museum of Comparative Zoology, and is 
taken as the type of this species. Two other specimens in 
Mr. Teller’s collection exceed this in size, one of them, meas- 
uring 11.5 cm. in a straight line, joining the extremities on a 
level with the triturating surface. The total length is estimated 
to have been about 14 cm. 
This species illustrates the differences between upper and 
lower dental plates most admirably, and is, in fact, the first in 
