THE 
AMERICAN NATURALIST 
Vöi. XXXIL July, 1898. No. 379. 
DENTITION OF DEVONIAN PTYCTODONTIDÆ. 
C. R. EASTMAN. 
THREE genera of Palæozoic Chimæroids, known only by re- 
mains of their dentition, constitute the, at present, imperfectly 
definable family Ptyctodontideæe. These are Ptyctodus, Rhyn- 
chodus, and Palæomylus, distributed throughout the middle and 
upper Devonian of Northern Europe and North America. The 
“jaws,” or dental plates as they are more properly called, are ` 
rarely well preserved, and invariably occur in the detached con- , 
dition. The solitary instance of four teeth associated in a 
group, as noted by Newberry in the type species of Rhyncho- 
dus, suggested the inference that in this genus, at least, the 
upper and lower dental plates were similar, each pair being 
directly united at the symphysis without the intervention of 
other teeth or plates. It has likewise been presumed that the 
dental plates of Ptyctodus were suturally united at the sym- 
physis, but the conformation of this region and distinctions 
between upper and lower jaws, or even between rights and 
lefts in the case of detached tritors, have not yet been made 
out. Neither have dorsal fin-spines, such as occur in most 
other Chimzroids, been positively established as belonging to 
this family. 
