318 CLASS REPTILIA. 



remains of the notochord occupy tlie iiitercentral spaces. The 

 neuro-central suture persists in many forms and it is common 

 to find separate cervical ribs. The cervical ribs are usually 

 double-headed, but the ribs of the thorax may be single or 

 double-headed. A lumbar region in wliich the ribs are indis- 

 tinguishable in the adult is usually present. In Sphenodon and 

 crocodiles the vertebral part of the ribs carry a posteriorly- 

 directed process, the uncinate process. The sacrum in living 

 forms, if present, nearly always consists of two vertebrae, but 

 is often more extensive in the extinct groups. It is absent in 

 Ophidia, Mosasauria and Ichthyosauria. Chevron bones (haemal 

 arches) are frequently present in the caudal region, and are 

 usually attached between the centra. Wedge-shaped inter- 

 central bones are sometimes found between the vertebrae of the 

 cervical and caudal regions. 



A sternum is very generally present (absent in Ophidia and 

 Chelonia). It is rhomboidal, and may be cartilaginous. The 

 ribs which reach it belong to the anterior part of the thoracic 

 region of the vertebral column, and the hindermost of them are 

 generally attached to a single or double backward prolongation 

 of it. Abdominal ribs are splint-like membrane bones placed hi 

 the ventral abdominal wall of Sphenodon, crocodiles, and some 

 extinct groups. They are of the nature of osteoderms and have 

 nothing to do with true ribs. 



In the skull there is a single occipital condyle {Monocondylea), 

 to which the exoccipitals usually contribute. In the auditory 

 region three bones are developed, the epiotic, opisthotic and 

 prootic. Of these it is characteristic that the epiotic unites 

 with the supra-occipital, and the opisthotic with the exoccipital, 

 before either of them unites with the prootic or with each 

 other. The prootic usually remains distinct. 



The basisphenoid bone is always present, and sometimes the 

 alisphenoid, but the presphenoid and orbito-sphenoid are usually 

 absent. The presphenoid is frequently replaced by a splint 

 (basisphenoidal rostrum) formed of membrane bone and pro- 

 jecting forwards from the basisphenoid. This splint represents 

 the anterior part of the parasphenoid of the Ichthyopsida, and 

 remnants of the posterior part of the same bone are sometimes 

 present on the ventral side of and fused to the basisphenoid. 

 The alisphenoid and orbito-splienoids are likewise sometimes 



