No. 374.] THE PRIMITIVE REPTILIA: 73 
considerable extent by the squamosal and prosquamosal bones, 
still shows to a considerable extent on the side of the skull; in 
the succeeding forms the quadrate is more and more reduced 
and the squamosal approaches more and more to an articulation 
with the lower jaw. Accompanying these changes are certain 
others, indicated below: 
Pelycosauria: Quadrate depressed, appearing on side of 
skull. Teeth simple. Two well-developed arches. 
Procolophonia: Quadrate depressed, nearly covered by the 
greatly enlarged quadratojugal (?). Teeth simple, reduced in 
number. Arches approximated only, a small foramen existing 
between the upper and lower. 
Cynodontia : Quadrate covered by supporting bones. Teeth 
showing small lateral tubercles. Arches more closely approxi- 
mated than in Procolophonia. 
Lycosauria: Quadrate small, covered by supporting bones. Skull 
depressed. Teeth with well-developed tubercles. Arches united. 
Gomphodontia: Quadrate very small, and inclosed in squa- 
mosal. Teeth tuberculate. Palate mammalian. Arches united. 
The author was at one time undecided as to the nature of the 
arch in the Lycosauria and Cynodontia; in connection with 
Baur he said (4) that the mode of formation of the arch was 
uncertain. A specimen of Cynognathus crateronotus, figured by 
Seeley, shows an opening between the upper and lower arches 
which was uncertain in origin, there being some reason to 
suppose it to be the result of an injury to the specimen, but a 
study of the figure of Procolophon, given by Seeley, shows the 
same condition. The enormous quadratojugal (called squamosal 
_ by Leydekker) joins the jugal in front, which in turn joins a 
slender element by its anterior superior corner; this element 
runs backward, forming the lower and back portion of the orbit, 
and is undoubtedly the postorbital. Behind this element is 
another bone, the squamosal, or squamosal + prosquamosal, 
which rests upon the quadratojugal below; between all these 
elements is a small cavity, exactly as in Cynognathus. It is 
hardly probable that a break would occur in the same place in 
the two specimens, and so they are considered as showing the 
final stages of the union of the two arches to form the mam- 
