SKELETON 



103 



palate, no parietal foramen, and the thecodont teeth are confined to the margins 

 of the jaws. In the complete series the gradual change of position of the choanas 

 can be traced from the oldest, in which they are beside the vomers; then in the 

 mesosuchia the palatines meet in the middle line, carrying the choanae back as 

 a single opening behind these bones; while in the recent species the pterygoids 

 have also met, so that the choanae are between them and the basisphenoid. 



Among the recent species (fig. 107) the basioccipital is excluded from the 

 foramen magnum, pre- and orbitosphenoids are imperfectly ossified, the nasals 

 are long and the 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 phylogenetic relation between the two, the skull of the 

 PteroS-AURS is very bird-like in its length and in having its axis at right angles 



Fig. 107. — Skull of Caiman latirostris, based on a figure by Reynolds; the irregu- 

 larities of the surface omitted, an, angulare; art, articulare; co, coronoid; d, dentary; 

 eo, exoccipital; /, lacrimal; mx, maxillary; />/, postf rental; pi, palatine; pmx, premaxillary 

 pi, pterj'goid; q, quadrate; qj, quadratojugal; sa, surangular; sq, squamosal; tr, trans- 

 versum; s, zygomatic, 



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. 



AV'ES. — The skull of birds is similar in many respects to that of lizards. 

 The chondrocranium (fig. 108) arises as two distinct parts, pre- and peri- 

 chordal, 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 behind 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 



