THE OPHIDIA. 235 



vertebrae. In tlie caudal region, elongated transverse pro- 

 cesses take the place of the ribs. Chevron bones, like those 

 of the Lacertilia, do not exist, but the caudal vertebrae pos- 

 sess bifurcated descending processes, which bear similar 

 relations to the caudal vessels. 



The skiiH differs from the ordinary Lacertilian cranium 

 in the following points : — 



1. That vertical elevation and lateral compression of the 

 presphenoidal region, which give rise to the interoi'bital 

 septum, is wanting ; the floor of the cranium being nearly 

 flat, and the vertical height of its cavity diminishing gra- 

 dually in front, so that it remains spacious between the 

 eyes, and in the frontal region generally. The periotic 

 region is not produced into parotic processes. 



2. The boundary-walls of the front half of the cranial 

 cavity are as well ossified as those of its posterior moiety, 

 and the bones which constitute the brain-case are firmly 

 united together. 



3. On the other hand, the nasal segment is less com- 

 pletely ossified, and may be moveable. The premaxillae are 

 usually represented by a single small bone, which very 

 rarely bears teeth. It is connected with the maxillae only 

 by fibrous tissue. 



4. The palatine bones never unite dii-ectly with the vomer, 

 or with the base of the skull, but they are usiially connected 

 with the maxillae by transverse bones ; and, by the ptery- 

 goids, with the mobile quadrate bones. Hence the connec- 

 tion of the palato-maxillary apparatus with the other 

 bones of the skull is always less close in Ophidia than in 

 LaceHilia, and sometimes it is exceedingly lax. 



5. The two rami of the mandible are united at the sym- 

 physis only by ligamentous fibres, which are often extremely 

 elastic. 



6. The hyoidean aj^paratus is very rudimentary, consist- 

 ing only of a pair of cartilaginous filaments, which are 

 united together in front, and lie parallel with one another 

 beneath the trachea. They have no connection with the 

 skull. 



