AND CLASSIFICATION OF THE FOSSIL REPTILIA. 
289 
from the nasal bones. Thus the suture becomes lost, not by the growth of the nasal 
at the expense of the pre-frontal, but by an absence of segmentation which causes the 
nasal region of such Chelonians to become Mammalian. In the same way I would 
interpret the loss of the post-frontal bone in Mammals. The common position of this 
bone is at the back of the orbital vacuity. It is manifest that in many Mammals 
there is no bone between the posterior border of the orbit and the jugal bone below, 
and in all such cases the bone may be lost through not being ossified. But in other 
types the post-orbital bar is present, and mainly formed by the frontal bone. It 
therefore would seem probable that the post-frontal had lost its individuality, because 
in the vertebral plan it was a portion of the arch formed by the frontal bones, just as 
the pre-frontals were portions of the arch formed by the nasal bones. The parietal 
bones in Lizards appear to show the accomplishment of a union of a similar kind. 
Theoretically there should be a pair of bones between the parietal and inter-parietal 
elements. These bones are not found, but the parietal is seen to bifurcate posteriorly, 
and the bifurcations have no obvious relation to the plan of the median element of 
the bone. There is some evidence, though very inconclusive, that these posterior 
arms of the parietals are separate ossifications in the Dicynodontia. They extend 
along the parietal crest, parallel to each other, overlapping the end of the parietal, 
and they appear to diverge forward, whereas the parietal bone of Dicynodonts is 
undivided. If this distinction should hereafter be established, it would contribute an 
element of symmetry in the theory of the skull, and would help to fortify the 
theoretical principles on which, in the matter of the bones referred to, a transition 
might be made from the Reptilian to the Mammalian type. 
A more important difference betwmen Reptiles and Mammals is found in the mode 
of union of the lower jaw with the skull. Theory has for a long time concerned itself 
with the fate of the quadrate bone. Sir Richard Owen, following the school of 
Cuvier, termed the quadrate bone the tympanic, and taught that it becomes in 
Mammals the ring which supports the drum of the ear. This is a view wdiich follows 
naturally enough from the study of the Clielonian skull; but I should never have seen 
my w^ay to accept it without the evidence which Anomodont skulls give of the history 
of the quadrate bone, and its relation to the squamosal. Hitherto those bones have 
been imperfectly understood. The squamosal is of large size and sends a zygomatic 
process forward, which combines wdth the malar bone to form the zygomatic arch, 
and it sends a process downward, in which the quadrate bone is embedded. I have 
here figured several examples of this relation of the quadrate. The squamosal extends 
in front of it and hides it, and extends internal to it, so that the lower jaw comes 
to articulate with the squamosal bone apparently, as well as with the quadrate. In 
one example the quadi’ate bone is perforated by a large excavation ; and I regard this 
excavation as auditory and comparable to the auditory notch or excavation in the 
Chelonian quadrate. In the Theriodont Galesaurus the form of the skull has become 
Mammalian ; the inferior process of the squamosal is lost, and the zygomatic process 
•2 P 
MDCCCLXXXIX. —-B. 
