REPTILES. 297 



chelonians the tympanic membrane is exposed ; in some lizards 

 it is partially covered by a flap developed from in front, while in 

 the crocodiles the flap is movable and the tympanum is some- 

 what sunken, the beginnings of the auditory meatus of the 

 higher vertebrates. 



In the skeleton the ossifications are far more extensive than 

 *H the amphibia. The notochord does not persist, except inter- 

 vertebrally in a few forms (geckoes and rhynchocephalia). The 

 vertebrae are usually precocious ; but amphicoelous vertebrae occur 

 in some or all theromorpha, ichthyosauria, plesiosaurs, rhyncho- 

 cephalia, geckoes, theropoda, orthopoda, and ornithopoda, while in 

 a few dinosaurs they are flat (amphiplatyan). In many groups 

 the neural arches are anchylosed to the centra, or again, as in 

 ichthyosaurs, turtles, and crocodiles, they are united by suture. 

 Haemal arches occur in snakes, lizards, and crocodiles. Trans- 

 verse processes, when present, are borne on the neural arch (i.e., 

 are diapophyses). 



At most five regions can be distinguished in the column ;. 

 but in the snakes, where no limbs are formed, only trunk and 

 caudal vertebrae can be distinguished. In the plesiosaurs axis- 

 and atlas are fused ; the proatlas of the crocodilia has beert 

 referred to (p. 143). Usually there are two sacral vertebrae. 



Ribs are usually present, and may be either with a single 

 head or bicipital. In the snakes they may extend the whole 

 length of the trunk with the exception of the atlas. In the 

 crocodilia and Hatteria thoracic and abdominal ribs are dis- 

 tinguished, the latter developing in the myocommata of the 

 ventral surface, and not extending to the vertebrae (see p. 147). 

 Cervical ribs are entirely lacking in the turtles, while in the 

 same group the thoracic ribs are united to the dermal plates 

 forming the carapace. 



A sternum is lacking in plesiosaurs, ichthyosaurs, turtles,, 

 snakes, and some snake-like lizards, while there is little evidence 

 as to its structure in the theromorphs and dinosaurs, where it 

 was apparently largely cartilaginous. When present it is tri- 

 angular or rhomboidal in outline, and contains no membrane 

 bone. In the flying reptiles (pterodactyls) it had a strong ven- 

 tral keel for attachment of the wing muscles. The episternum 



