478 THE AMERICAN NATURALIST. (VoL. XXXII. 
paper on the State Quarry fauna already referred to! represents 
an average-sized individual 4.5 cm. in length. Upper dental 
plates (Figs. 12, 13), as far as their characters can be made out 
from the meager material at hand, do not differ materially from 
those described below as P. ferox, excepting, of course, that 
they are smaller, and are conformable to the lower jaw in curva- 
ture. The following remarks are, therefore, to be understood 
as applying exclusively to the lower dental plates. 
Viewed from above, the curvature of the lower dental plates 
is seen to be more or less sigmoidal, the median line being in 
the left ramus -shaped, and in the right \-shaped. The outer 
face is usually straighter. and more nearly vertical than the 
inner. Very frequently the bony tissue enclosing the tritor is 
thickened so as to form a slight convexity on the inner poste- 
rior face, and its outline sweeps around posteriorly as an inde- 
pendent curve beyond the median line of the tritor, until it 
finally becomes merged with the less-rounded outer face of the 
jaw. The intersection of these curved outlines forms superi- 
orly a peaked ridge just behind the tritoral area (Figs. 3, 16, 
17) ; and it is to be noted that this ridge always lies externally 
to the median line of the tritor, or, to express it differently, 
the tritoral area tapers posteriorly toward the inside wall of the 
jaw, and is nearer to that side than the outer. 
The tritoral area occupies nearly the full width of the upper 
surface of the jaw and partakes of the same curvature. Start-. 
ing from behind, it curves first inwardly for about one-half its 
length, and then reverses this direction so that the anterior 
extremity tapers outward, and as the more arcuate boundary 
obviously lies on the inner face of the jaw, we are furnished 
with a convenient clue to the orientation in the case of detached 
tritors. In general, the parallel laminz, or rows of punctate 
which indicate them superficially, are directed forward and 
inward, but exceptions to this rule are not uncommon, owing 
to irregularities in the arrangement of the medullary canals 
and inequal wearing away of the triturating surface. The 
latter cause is a powerful determinant in affecting the superior 
aspect of the tritors. 
1 Ann. Rep. Towa Geological Survey, vol. vii (1897), p- 115, Fig. 10 4. 
