194 
PROFESSOR H. G. SEELEY ON THE STRUCTURE, ORGANIZATION, 
orbital vacuity as probably nasal, and consequently the dentigerous bone as probably 
premaxillary, though the morphological data for the identification are confessedly 
slender. 
The premaxillary hones. —This bone resembles the same element in Ichthyosaurus 
in steadily augmenting in depth as it extends backward. Its upper hinder margin 
is notched out by the vacuity which I am disposed to regard as nasal. The form 
of the bone is a long triangle with its narrow base towards the orbit. Its depth 
posteriorly, as exposed by the removal of the covering nasal bone, is about one centim. 
Its length as preserved is between 3 and 5 centims. There is a longitudinal groove 
above the bases of the teeth, like that seen in Belodon, and which indents the 
Ichthyosaurian jaw parallel with the base of the dental groove. The rough convex 
surface of bone between this groove and the alveolar border has been removed along 
its length, apparently to expose cavities like sockets which may have been for 
successional teeth, of which 18 are visible. Although these pits existed beneath the 
teeth which were in use, there is no evidence that those teeth were in sockets. The 
teeth were manifestly anchylosed to the jaw as in Lybyrinthodonts and some Lizards. 
A horizontal plate appears to have divided the base of the teeth from the quadrate 
cavity beneath. One tooth appears to be in one of these sockets. The teeth were 
closely set, but are nearly all wanting, and only indicated - by the infra-dental cavities 
and by impressions of the crowns.' There is no trace of successional teeth in any 
other of these infra-dental spaces. They are uniform in size and depth, and in most 
cases, but not always, immediately beneath the crowns. They are not circumstanced 
like cavities for successional teeth so far as these are known, and are apparently 
interior in position to the teeth on the alveolar margin. The crowns of the teeth 
appear to have been smooth, conical, pointed, with the base circular. One of the 
longest, in front, measures 5 millims. from the point to its anchylosis with the jaw, 
and about 7 millims. to the bottom of the infi'a-dental cavity. 
The maxillary hone .—The posterior part of the dentigerous border may be a 
separate bone, but if so the suture which defines the maxillary bone is not clearly 
made out. It probably is in front of the last two infra-dental cavities, above a 
depression which indicates a squamous overlap upon the premaxillary bone. As it 
extends backward below the orbit, three or four slender pointed teeth are seen to 
extend from it, but more may be hidden in the matrix. The bone terminates 
backward in an oblique suture which is below the middle of the orbit, and therefore 
presumably indicates the jugal bone, which is imperfectly exposed and apparently 
displaced downward. 
Above the maxillary region the cervical vertebras lie over the orbit and the back 
of the head. The violence which separated the vertebral column disengaged the 
lower jaw and separated its elements, and displaced the quadrate bone and bones of 
the palate, which lie scattered between the head and the lower jaw, not entirely free 
from matrix. 
