188 THE ANATOMY OF YERTEBRATED ANIMALS 



appendage is prettj certain to part at one of these weak 

 points. 



The arches of the vertebrre of the Lacertilia are articulated 

 together by the ordinary oblique processes, or zygapophyses. 

 In the Iguance they are additionally connected by a process 

 of the front part of each arch {zygosph&ne)^ which fits into a 

 fossa on the posterior face of the preceding arch {zygantrum). 

 These Lacertilian vertebras thus nearly ajDproach the vertebrae 

 of the Ophidia, 



The transverse processes of the vertebrae are very short, 

 and are, at most, divided into two indistinct facets, with which 

 corresponding facets on the proximal ends of the ribs articu- 

 late. 



Ribs may be developed in all the cervical vertebrae except 

 the atlas, and they usually increase in length toward the dor- 

 sal region, where more or fewer of them become connected 

 with the sternum. The dorsal moiety of the primitive carti- 

 lage of the rib becomes ossified, and the primitive cartilage- 

 bone is finally replaced by membrane-bone. The ventral moi- 

 ety becomes converted only into cartilage-bone, and may pass 

 directly and without articulation, on the one hand into the 

 sternum, and on the other into the vertebral rib. Processes 

 are sometimes developed from the posterior margins of certain 

 of the ribs, which are termed p>'^^ocessus iincmati. The ster- 

 num, when fully formed, consists of a rhomboidal anterior por- 

 tion, from the posterior angle of which a single, or double, 

 backward prolongation is continued into the wall of the abdo- 

 men. Two or three pairs of the sternal ribs are connected 

 with the posterolateral edges of the rhomboid, while the rest 

 may be attached to the abdominal prolongations ; or, behind 

 these, they may be continued into one another, to form com- 

 plete hoops across the wall of the abdomen (Geckos, Chamae- 

 leons, Scincoids). 



The Flying Lizard {Draco volans) is remarkable for the 

 elongation of many of its posterior ribs, which are continued 

 into, and support, the parachute-like expression of the integu- 

 ment by which it is enabled to perform its flights. 



The skull of the Lacertilia resembles that of the Chelonia 

 in the development of an interorbital septum (except in the 

 Amphishcence)^ and in the absence of alisphenoids, or of any 

 complete ossification of the presphenoidal or orbito-sphenoidal 

 regions. The premaxillae and maxillae are firmly united with 

 one another and with the skull, and there are two vomers. An 

 unossified space, the^;a?7*e^«^ybra»?e;i, usually remains in the 



