REPTILIA. 383 



Crocodilia, in Hatteria and Chamaeleons among Lacertilia, and in the 

 extinct Icthyosauria and Plesiosauria. The bones of the lower jaw usually 

 retain their sutures : but in Ophidia the articular, coronoid, angular and 

 surangular are anchylosed. The rami are anchylosed at the symphysis in 

 Chelonia ; connected by a distensible ligament in Ophidia, or united by 

 suture, or cartilage. Iguanodon, Hypsilophodon and Diclonius among Deino- 

 sauria, possess a prae-symphysial or mento-meckelian bone in front of the 

 symphysis. The stapes (columella auris) is a simple rod-like bone. The 

 hyoid arch is well-developed and the Chelonia have a large remnant of the 

 first branchial arch. 



Amphicoelous vertebrae are found in the existing Hatteria and 

 Geckoes among Lacertilia : in the prae-cretaceous Crocodilia, e. g. Teleo- 

 saurus : in Ichthyosatiria, Plesiosauria and many Deinosauria. In these the 

 notochord is enlarged intervertebrally. In other Reptilia it is enlarged 

 intra-vertebrally and constricted inter- vertebrally and eventually lost. The 

 centra are usually precocious and are connected by synovial joints, or in 

 the Crocodilia by intervertebral discs. Opisthocoelian or biconvex centra 

 are found in exceptional instances, concavo-convex in many Deinosauria ; 

 and the mobility of the neck in Chelonia is insured by a great variety in the 

 shapes of the centra of the cervical vertebrae. The neural arches are united 

 to the centra by anchylosis in Ophidia, Lacertilia and most Chelonia', by 

 suture in a few Chelonia, Crocodilia, the extinct Plesiosauria ; and by inter- 

 posed cartilage in the Ichthyosauria. In the last named order, the vertebrae 

 articulate only by their centra ; in the remaining orders by the usual arti- 

 culating processes, and in Ophidia, as well as in the Iguanidae among 

 Lacertilia by a zygosphene and zygantrum in addition (see p. 73). In 

 Ophidia, in Amphisbaenoidea among Lacertilia and the Ichthyosatiria the 

 vertebral column is divisible into a praecaudal and caudal series (see 

 p. 72) : in other orders into a cervical, dorsal, lumbar, sacral, and caudal 

 regions, but the lumbar region is absent in Chelonia. The number of verte- 

 brae in each region varies much in the different orders. There are in living 

 Reptilia two sacrals, but the number may be greater among extinct 

 forms, e.g. 3-6 in Pterodactyles, 4-6 in some Deinosauria. In the Ichthyo- 

 sauria the atlas and axis resemble the remaining vertebrae in their general 

 form. They are often anchylosed in Plesiosauria and in Cretaceous Ptero- 

 dactyles. The atlas in most Reptilia consists of a ventral body and two 

 supero-lateral arches, to which is added in the Crocodilia a superior or 

 dorsal membrane bone : and the axis has a well-developed odontoid 

 process united to it by cartilage, by suture in Crocodilia, or by anchylosis 

 as in Lacertilia. The caudal vertebrae in many Lacertilia have a transverse 

 unossified septum through which the tail is apt to split. 



Ribs are always present. Their proximal ends though possessing two 

 articular surfaces may be simple, e. g. Ophidia, Lacertilia, or deeply divided, 



