i6o 



VERTEBRATE SKELETON 



sively fused than in other Squamata, there being but four separate 

 bones in adult rattlesnakes and some other poisonous species, and 

 five at most in other forms. The halves of the lower jaw are con- 

 nected in front by an elastic ligament, permitting a wider opening of 

 the mouth. The hyoid is represented by a curved cartilage bar or 

 is absent. 



Fig. 1 71. — Cranium of Python reliculatus. bo, basioccipital; bs, basisphenoid; /, 

 frontal; w, maxilla; n, nasal; pa, parietal; pi, palatine; pm, premaxilla; pof, prj, post- 

 and prefrontals; pt, pterygoid; q, quadrate; r, rostrum; :?, supraorbital; so, supraoccipital; 

 sq, squamosal; s -\- v, septomaxillary and vomer; /.transversum. 



RHYNCHOCEPHALIA.— This group, represented today only 

 by Sphenodoti, is related to the Lacertilia, the chief distinctions being, 

 so far as skull is concerned, a fixed quadrate and the presence of all 

 three temporal fossae. 



These lacertan resemblances exist in the chondrocranium, but the cartilages 

 are more extensive and the prechordal part arises separately from, and at an 

 angle to the posterior portion (fig. 172), the flexure being lost later. The basal 

 plate is entire and is connected with the otic capsules, the medial walls of which 

 long remain open. The sphenolateral cartilage is connected with the basal, 

 and the interorbital septum supports supraseptal plates. 



At a later stage the synotic tectum, formed entirely by growth from the otic 

 capsules, sends a bar forwards which surrounds the parietal eye. The spheno- 

 lateral becomes connected with the supraseptal by marginal and lateral teniae, 



