AND CLASSIFICATION OF THE FOSSIL REPTILIA. 
193 
raised and transversely roughened. The least transverse measurement across the 
frontal bones at the middle of the orbital concavity was 11 millims. In the median 
line, behind the middle of the orbits, the frontal bones form a slight longitudinal 
median ridge, anterior to which a wide shallow median concavity extends forward, 
and is prolonged down the upper surface of the nasal bones. 
Prefrontal and lachrymal bones .—There appears to be a slight channel above the 
anterior border of the orbit, which is increased by a slight displacement of the 
prefrontal bone. This bone forms the anterior border of the orbit. It widens as it 
extends forward and downward from the middle of the orbital border in the frontal 
bone to the dentigerous bone, which for the present may be premaxillary or maxillary. 
This bone is 15 millims. long by about a centim. wide where widest, in its lower third. 
It extends under the frontal bone above, and overlaps the dentigerous bone below. 
A suture divides it transversely, so the lachrymal bone is present as a separate 
ossification. In front of the lachrymal bone is a notch, which also indents the upper 
hinder part of the bone which carries most of the teeth. This foramen is led up 
to by a longitudinal channel in the dentigerous bone. The nasal bones would have 
reached the superior border of this foramen. Hence it is evidently an ant-orbital 
vacuity, but whether it is comparable to the ant-orbital vacuity of Teleosaurs, 
Dinosaurs, Ornithosaurs, and Birds, or to the ant-orbital vacuity of Ichthyosaurs, 
which is similarly placed, and forms on each side of the head the anterior narine, 
depends upon the interpretation of the bones which form its anterior borders. 
The nasal bones .—-The nasal bones roof over the head in front of the orbits, and 
are united by suture with the frontal bones behind. They are imperfect anteriorly, 
but as preserved are 3 centims. long. They are united by a median straight 
longitudinal suture, and form a shallow longitudinal concavity extending forward on 
the snout. They have a transverse width of 12 or 13 millims. posteriorly, and narrow 
anteriorly to a width of 4 or 5 millims. at the anterior fracture. Laterally, each bone 
makes an angular bend downward, so as to overlap and make a squamous union with 
the long dentigerous bone which runs parallel to it and forms the toothed margin of 
the jaw. 
The question whether that bone is premaxillary or maxillary may now be examined. 
If the converging borders of the nasal bones were prolonged anteriorly, they would 
terminate one centim. in advance of the fracture, or half a centim. from the extremity 
of the jaw. Hence it is probable that if the nares were terminal they were small, 
though not smaller than in some Lizards. The large nasal bones, however, are not 
Lacertilian, and find no parallel so close as may be seen in Ichthyosaurus. And 
then the dentigerous bone would closely resemble the premaxillary bone in those 
Ichthyosaurs in which the nasal bones extend to near the end of the snout. A 
corresponding elongation of both nasal and maxillary bones is seen in Crocodiles ; but 
the anterior groove, which in Protorosaurus runs up to the ant-orbital vacuity, is 
similar to that seen in Ornithosaurs and Birds ; and this leads me to regard the ant- 
MDCCCLXXX VII.—B. 2 C 
