SKULL REPTILES 



149 



All of the typical bones appear in the development of the lower 

 jaw, sometimes remaining distinct in the adult. The halves of the 

 jaw are ankylosed in the adult. The hyoid body, perforate in 

 Chelys, is more or less elongate and bears a weak entoglossal process, 

 possibly the remnants of a hyoid-mandibular copula. The body 

 may persist wholly as cartilage or it may ossify from one or three 

 centres, the pair in the latter case being anterior. The hyoid cornua 

 sometimes persist as cartilage, when ossified they are fused with 



{A) (B) 



Fig. 158. — Hyoid apparatus of (^4) Chelone (Goppert), and {B), Trionyx. Cartilage 

 stippled, ar, arytenoid; b, branchial arches; bh, basihyoid; d, dilator muscles; g, glottis; 

 h, hyoid; he, hyoid cornu; sph, sphincter muscles; tr, trachea. 



the body. The first branchial is ossified, at least in part, and both it 

 and the second arch may be entire or subdivided into separate 

 elements. 



Sauropterygia have synapsidan skulls (fig. 159), with a large temporal 

 fossa, bounded laterally by squamosal and a large bone, probably united post- 

 frontal and postorbital, the zygomatic being excluded from the border and no 

 quadratojugal being present, the zygomatic articulating with the squamosal. 

 Exoccipitals sometimes form part of the occipital condyle. A parietal foramen 

 is present between the separate parietals, each of which sends a process laterally 

 to the squamosal, bounding the temporal fossa behind. Parietal and frontal of 

 a side are frequently fused to a fronto-parietal, while a prefrontal excludes the 

 frontal from the orbit. Sclerotics are absent. The orbit is close to the naris, 

 the latter bounded by prefrontal, maxilla and premaxilla. The quadrate, 



