SKELETON. 95 
premaxillaries short so that the nares are far in front; parietals and usually the 
frontals are fused in the middle line. There are vacuities in both walls of the 
lower jaw, which is also pneumatic. 
Although there is no relation between the two, the skull of the Prerosaurs is 
very bird-like in its length and in having its axis at right angles to that of the body, 
while the elongate premazxille form a bird-like beak. The sutures between the 
bones are largely obliterated in the adult and the brain cavity recalls that of birds. 
The resemblances are heightened in some by the lack of teeth, in others they are 
in‘sockets. Both supra- and infratemporal fosse are present, as well as a large 
preorbital vacuity, sometimes united with the naris. Squamosal and quadrate 
are inclined forward so that the hinge of the jaw is often beneath the orbit. There 
is no parietal foramen and all of the bones of the jaw are fused, including those of 
the two halves. 
Fic. 98.—Skull of Caiman latirostris, based on a figure by Reynolds; the irregularitesi of 
the surface omitted. Letters as in fig. 68. 
CKo@ebir eB) 
AVES.—The skull of birds is similar in many respects to that of lizards. The 
chondrocranium arises as two distinct parts, pre- and perichordal, which, on account 
of the great head flexure, are at an angle of 100° to each other, later increased to 
160°, which persists through life. ‘There are three (or four?) occipital vertebre be- 
hind the ear, the last being the most prominent, and there is a small synotic tectum. 
From the first the otic capsules are continuous with the basal plate and the fenestra 
vestibuli is formed later by resorption of the cartilage. The trabecule are at first 
distinct from each other as well as from the perichordal part; later they fuse in 
front of the hypophysis to give rise to the base of the interorbital septum. In 
Tinnunculus the ethmoid plate arises early as an intertrabecular mass, from which, 
later, the dorsal part of the interorbital septum arises as a backward growth of 
cartilage. Large alisphenoid cartilages are connected with the otic capsules. 
The nasal capsules are complicated and later give rise to several centres of ossi- 
fication. The quadrate is free from the rest (streptostylic) and its pterygoid process, 
the homologue of the pterygoid cartilage in other groups, is greatly reduced. The 
other visceral arches are much as in the adult (injra). 
