546 THE AMERICAN NATURALIST. (Vou. XXXII. 
blance. Thus, while there is a general homogeneity of type in 
Ptyctodont dentition, transitional stages are to be observed in 
its different expressions. 
Paleomylus frangens and P. crassus are sufficiently well known 
through Newberry’s figures and descriptions. With P. greenez, 
however, the case is not so fortunate. It has not been hitherto 
illustrated, and the original description is very brief. As stated 
by Newberry, it resembles the type species (P. frangens), but 
differs in being narrower vertically, and longer and much 
thicker at the anterior border. No distinctions are pointed out 
between upper and lower dental plates. 
For further information with regard to P. greenei we have to 
thank Messrs. Teller and Monroe, of Milwaukee, who have 
kindly supplied all the material in their possession. Of this, 
the most remarkable specimen — and, we may safely say, one of 
the most important examples of Ptyctodont dentition yet dis- 
covered — is that photographed on the accompanying plate 
(Fig. 48). It is rivaled only by the specimen of Rhynchodus 
secans described by Newberry, already referred to, in which 
four teeth were found associated in a group; and the two taken 
together prove beyond a doubt that the dentition consisted of a 
single pair of dental plates in both upper and lower jaws. The 
present specimen was obtained by Mr. Teller in the vicinity of 
Milwaukee, and is preserved in his private collection. 
The teeth are imbedded in a block of limestone measuring 
35 cm. in its greatest length, which coincides with the longitu- 
dinal axis of the jaws. The illustration may be most conven- 
iently examined by turning the page sideways, with the bottom 
on the left, and top on the right-hand side of the observer. 
Oriented in this position, the two lower dental plates will be 
found on the right-hand side of the figure, in advance of the 
upper pair; the left upper dental plate is immediately above 
the right upper, and the left lower above the right lower. All 
plates have the external surface exposed, with the exception 
of the right upper, which is broken through obliquely. The 
inner side is seen near the anterior beak, but farther back it 
is beveled down through its entire thickness, leaving only an 
impression of its outer surface on the matrix. Still it shows. 
