548 THE AMERICAN NATURALIST. [VoL. XXXII. 
tooth. The outlines are preserved nearly intact as far as the 
extreme antero-superior portion, including the beaks, where 
there is a slight deficiency. The fracture being an uneven one 
at this point, and likely to present a misleading appearance, a 
strip was filled in with plaster up to a level with the external 
surface as far as the impression of bone substance was preserved 
on the underlying matrix, but no further. The original boundary 
was probably not far distant from the dotted line shown in the 
figure, which has been restored from the outlines of other 
specimens. 
The differences between upper and lower dental plates are 
not nearly so decided as in either Ptyctodus or Rhynchodus, 
yet, such as they are, leave no reasonable doubt as to the posi- 
tion occupied by the several teeth in the mouth. The lower 
dental plates have a more pronounced anterior beak, and are 
also deeper vertically than the upper pair; and the triturating 
surface is more uneven. The outline of the latter is sinuous, 
there being an anterior and a posterior depression, separated 
by a median elevation ; and there are corresponding, although 
gentler, undulations to match in the margin of the upper dental 
plates. The opposing outlines coincide most nearly with one 
another when we make the upper beaks protrude slightly in 
advance of the lower, exactly as was done in the case of RAyn- 
chodus secans. But there is no evidence that the beaks of the 
lower jaw closed outside the upper dental plates, as in Ptyctodus. 
On the contrary, appearances indicate that the two jaws came 
into direct opposition with their triturating surfaces, the same | 
as molar teeth. There is a reverse slope to the grinding surface 
in both jaws ; the anterior depression has a decided slope down- 
ward and inward, and the posterior depression an equally pro- 
nounced one downward and outward. The grinding surface has 
an average width of about 1.5 cm., and extends from the beaks 
as far back as the supero-posterior angle, or where it meets 
the perfectly straight line forming the posterior margin. 
There is a peculiar appearance about the beak of the left 
upper dental plate which deserves notice, although it challenges 
explanation. Owing to its faulty state of preservation, nO 
very satisfactory conclusions can be formed as to its nature OF 
