202 THE ANATOMY OF YERTEBRATED ANIMALS. 



The transverse processes are short and tubercle-like, and 

 the heads of the ribs which articulate with them are simple. 

 Each rib usually gives off a short upward process at a little 

 distance from its head ; it is curved, usually hollow, and ter- 

 minates, inferiorly, in a cartilage which is always free, no 

 trace of a sternum existing. Strong descending processes are 

 given off from the undersides of many of the presacral verte- 

 brae. In the caudal region, elongated transverse processes 

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

 Lacertilia^ do not exist, but the caudal vertebrae possess bifur- 

 cated descending processes, which bear similar relations to 

 the caudal vessels. 



The skull 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 interorbital sep- 

 tum, are wanting ; the floor of the cranium being nearly flat, 

 and the vertical height of its cavity diminishing gradually 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 cav- 

 ity are as well ossified as those of its posterior moiety, and the 

 bones which constitute the brain-case are firmly united to- 

 gether. 



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

 ly ossified, and may be movable. The premaxillse 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 directly with the vomer, 

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

 with the maxillae by transverse bones ; and, by the pterygoids, 

 with the mobile quadrate bones. Hence the connection of the 

 palato-maxillary apparatus with the other bones of the skull is 

 always less close in Ophidia than in Lacertilia^ and some- 

 times 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 apparatus is very rudimentary, consisting 

 only of a pair of cartilaginous filaments, which are united to- 

 gether in front, and lie parallel with one another beneath the 

 trachea. They have no connection with the skull. 



These are the most apparent differences between the 



