No. 379.] DEVONIAN PTYCTODONTIDE. 481 
which these distinctions have been made out. The tritoral 
area of the upper jaw terminates anteriorly in a slight promi- 
nence or tubercle, situated somewhat nearer the outside than 
the inside face, and from this point onward as far as the crest 
of the symphysis a decided beveling is seen along the outer 
face (Fig. 35) where the beak of the lower jaw played against 
it. With increasing wear the beveling becomes converted into 
an excavation, the appearances suggesting that the jaws were 
movable, not only in a vertical, but = to some extent in an 
antero-posterior, direction. 
Anteriorly, the upper dental plates project forward and 
upward in a gently curved line, obviously for the purpose of 
strengthening the symphysial attachment, and they are also 
bent inwardly toward the front. As the inner face is but 
slightly thickened at the symphysis, the upper dental plates 
must have been more closely approximated anteriorly than the 
lower, thus permitting the prehensile beaks of the latter to_ 
pass by them on the outside, and allowing the cutting edge to 
close like the blade of a pair of shears. It is also plain that 
the rami of both jaws must have met at a rather acute angle 
anteriorly, forming a narrow V. The upper dental plates have 
a slight sigmoidal curvature, the posterior end flaring out and 
the symphysial portion being inflected inward. 
The lower dental plate in this species is remarkable chiefly 
for its great height along the anterior margin, general straight- 
ness in an antero-posterior direction, and powerful prehensile 
beak. Superficially, it is marked by fine concentric striæ 
similar to those in Palæomylus, directed more or less parallel 
to the triturating surface and running at right angles to the 
second set commonly found in species of Ptyctodus. Four 
examples of the lower dental plate have come under the writer's 
observation, one of which is unusually heavy and corresponds 
in size to the largest of the upper dental plates. A medium 
and a small-sized individual are shown in Figs. 36, 37, and 39, 
the last two presenting the inner and outer aspects, respectively, 
of the same specimen. The original of Fig. 36 was found by 
Mr. C. E. Monroe near Milwaukee, and is preserved in his 
private collection, It represents a comparatively young indi- 
