338 



ZOOLOGY 



SECT. 



alisphenoidal elements are absent, their places being taken by 

 downward prolongations of the parietals and frontals. In the 

 substance of the mesethmoid are two cartilaginous tracts (Fig. 

 1000, B, T) which are the persistent trabeculae of the embryonic skull. 

 The inter-orbital septum is absent, and the cranial cavity is pro- 

 longed forwards to the ethmoidal region. Neither upper nor 

 lower temporal arches are present. The palatines (PI) are movably 

 articulated with the base of the skull ; as in the Lizards, they are 

 widely separated from one another, and do not develop palatine 

 plates. They are movably articulated behind with the pterygoids 

 (Pt), and the latter, through the intermediation of the slender 



transverse bones 

 (Ts), with the 

 maxillae. The pre- 

 maxillae are very 

 small (in some 

 venomous Snakes 

 entirely absent), 

 and when present 

 usually fused to- 

 gether. The 

 maxillae (Mx), usu- 

 ally short, articu- 

 late by means of 

 a movable hinge- 

 point with the 

 conjoined lacry- 



FIG. 1000. A, lateral view of skull of Rattlesnake (Crotalus). ? a . n , P J 6 " 



B. 0. basi-occipital ; B. S. basi-sphenoid ; E. 0. exoccipital ; I T O n t a 1 (La), 

 F. O. fenestra ovalis ; La. conjoined lacrymal and pre-frontal ; 

 L. f. articulation between lacrymal and frontal ; Mn. mandible 

 MX. maxilla; Na. nasal; PI. palatine; Pmp. premaxilla ; 



P. Sph. presphenoid ; Pt. pterygoid ; Qu. quadrate ; Sq. squa- 

 mosal ; //, V, foramina of exit of the second and fifth cranial 

 nerves. B, transverse section at point lettered B in Fig A ; T 

 trabeculae. (After Huxley.) 



in turn, is 

 movably con- 

 nected with the 

 frontal. The long 

 and slender quad- 

 rate (Qu) is freely articulated with the posterior end of the elongated 

 squamosal. The rami of the mandible, likewise long and slender, 

 are not united anteriorly in a symphysis, but are connected together 

 merely by elastic ligamentous tissue, so that, when the mouth of 

 the Snake is opened to allow of the entry of the relatively large 

 prey, which it swallows whole, they are capable of being widely 

 separated from one another. The Typhlopidae differ from the 

 rest of the Ophidia in having the maxillae immobile, the quadrate 

 more closely connected with the skull, and the rami of the mandible 

 united by a fibro-cartilaginous symphysis. 



The skull of Sphenodon (Fig. 1001) differs very considerably 

 from that of the Lizards. There is a large supra-temporal fossa 

 bounded by the parietal, post-orbital (Pt. f), and squamosal, and 



