54 corK 



element from the opisthotic. In the Lacertilia it is ex- 

 cluded from the embrace of the petrosal and exoccipital, 

 and is carried on their extremity, and is in contact with 

 the quadrate. In the Serpentes, in consequence of the 

 loss of the arches, the quadrate is borne on the extremity 

 of this paroccipital, which generally projects freely from 

 the cranium. 



The vertebrae of Reptilia are either biconcave (amphi- 

 coelous), or flat at both extremities (amphiplatyan), or ball 

 and socket. The latter is of two types : first, with the 

 socket in front and the ball behind or procoelous, which is 

 the most frequent ; or second with ball in front and socket 

 behind, or opisthocoelous. Intervertebral articulations 

 other than those of the zygapophyses are found ; as the 

 zygosphen and zygantrum, (Serpentes, Fig. 29, p. 81), and 

 the hyposphen and hypantrum (Dinosauri Saurischia, Fig. 

 22, p. 69), and some others. 



In the scapular arch, a sternum, proscapula, presternum, 

 interclavicle and clavicle may be present or absent. Scap- 

 ula and coracoid are always present, and usually a pre- 

 coracoid. 



Twelve orders of Eeptilia are known : 



I. The quadrate bone immovably fixed to the ad- 

 jacent elements by suture. 



A. Scapular arch external to ribs ; temporal region 

 with a complex bony roof; no longitudinal post- 

 orbital bars. 



A tabular and supramastoid bones, and a 

 presternum; limbs ambulatory; ver- 

 tebrse amphicoelous ; (1) Cotylosauria. 



AA. Scapular arch internal to ribs ; temporal region 

 with complex roof and no longitudinal bars. 



A presternum ; limbs ambulatory ; (2) Ghelydosauria. 



