72 THE AMERICAN NATURALIST. [VOL XXXII. 
bones. They seem to form a definite group, with this feature 
as the common point in their structure. The quadrate is not 
equally depressed in all forms, nor equally surrounded by the 
bones of the temporal region. Thus the ancestors of the 
Pelycosauria were in all probability forms lacking the elongate 
neural spines characteristic of this group, with the quadrate 
distinct from the surrounding elements and not so much 
depressed. These forms are as yet unknown. The Pelycosauria 
are no longer considered as a side branch of the main reptilian 
line, but as one member or branch of an equally divided line 
leading from the Proganosauria. 
From this point onward the Reptilia are divided into two 
groups, one with an elongate quadrate which includes all 
modern and most extinct Reptilia, and one with a depressed 
quadrate reaching its highest development in the Permian, and 
in all probability losing its identity by almost imperceptible 
stages in the direct ancestors of the Mammalia. 
Haeckel, in his Systematische Phylogenie (Vertebrata), page 
299, has grouped the Permian forms under two orders, the 
Theriodontia (Jastocephale theromoren) and the Anomodontia 
(Chelycephale theromoren). In the first order he places the 
suborders Pareiasauria, Pelycosauria, and Palatosauria; in sec- 
ond the Dicynodontia and Udenodontia. If it be true that 
the Pareiasauria are a distinct order they must be dropped 
from this group. Then the remaining suborders, as defined by 
Haeckel, comprise the forms possessed of the depressed quad- 
rate. It has been shown by Baur and Case (4) that the group 
Theromora does not exist, and it is now suggested that the forms 
with the depressed quadrate be referred to as the mastocepha- 
lous Reptilia, because of their evident culmination in the Mam- 
malia, while the remaining Reptilia may be described as sauro- 
cephalous. In no known form, so far as I am aware, is there a 
tendency for a member of one of these groups to assume the form 
and condition of the quadrate characteristic of the other. 
Leaving out of consideration the aberrant Dicynodontia 
and Udenodontia, a steady progress can be traced from the 
primitive pelycosaurian forms to the mammal-like forms. The 
quadrate of the early forms, while flattened and covered to a 
