198 REPTILIA. 



unpaired. The frontal bone in many cases takes no part in the 

 formation of the roof of the cranial cavity, and only lies on the 

 interorbital septum. Behind the lateral parts of the frontal in 

 the temporal region are the postfrontals (Pf). In the ethmoidal 

 region the median part remains in part cartilaginous, and is covered 

 above by the paired nasal bones (N), and at the base by the vomer 



* (Fo), which in the Snakes and Lizards is paired. The lateral parts 



are always separate from the median, and are known as the lateral 



ethnioids or preef rentals (Prf). In the Lizards and Crocodiles 



( lachrymals (L) are present on the outer side of the prsefrontals, 



bounding the anterior margin of the orbits. 



The squamosal (Sgr) is more intimately applied to the cranium, and 



y the quadrate ($) is always a strongly developed bone. In Chelonia 

 and Crocodilia the quadrate and maxillo -palatine apparatus are im- 

 movably united with the wall of the skull ; in Snakes and Lizards,. 

 on the other hand, they are more or less freely movable. In the 

 first case not only are the large pterygoid and palatine bones fused 

 with the sphenoid, but the quadrate bone is very firmly connected 

 with the superior maxillary (i.e. jugal) arcade. In the Crocodiles a 

 transverse bone (os transversum} is developed between the pterygoid 

 and maxillary, and also a superior temporal arcade by which the 

 squamosal is connected with the postfrontal on either side. In the 

 Lizards, in which the maxillo-palatine apparatus and quadrate are 

 movably articulated to the skull, the jugal arch is completely absent 

 [i.e., the jugal is not connected with the quadrate by bone]. On the 

 other hand, these animals possess not only a transverse bone (os 

 transversum) (fig. 631, Tr\ already mentioned for the Crocodiles,, 

 but also a column-like bone the columetta which extends between 

 the parietal and pterygoid. The facial bones are, however, most 

 movable upon one another in the Ophidia, which are without the 

 jugal arcade, but present a large os transversum. The two rami of 

 the mandible, which in all Reptilia and lower Vertebrates are com- 

 posed of several pieces, are in these animals connected at the symphysis 

 by an elastic band, an arrangement which permits of considerable 

 extension towards the sides. 



The visceral skeleton is reduced to the hyoid bone, from the anterior 

 arch of which the dorsal element (hyomandibular) is separated off, 

 enters into relation with the auditory apparatus, and is known aa 

 the columella. The hyoid bone is most reduced in the Snakes. 



The limbs and their girdles are completely absent in most Snakes. 

 In the Peropoda and Tortricidce, however, traces of hind limbs are 



