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 PTEROSAURS is 

 very bird-like in its length and in having its axis at right angles to that of the body, 

 while the elongate premaxillae 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 fossae 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. 



FIG. 98. Skull of Caiman laiirostris, based on a figure by Reynolds; the irregularitesi of 

 the surface omitted. Letters as in fig. 68. 



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 vertebrae 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 trabeculae 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 

 Tinmmculus 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 (infra). 



