338 



CLASS REPTILIA. 



Mst. 



spots caused by these unossified septa. The lost tail is replaced by 

 regeneration, but the new tail (which may be double) has an imperfect 

 skeleton and its scaling may differ from that of the lost part.* 



The ribs are single headed and are attached to the vertebrae 

 between the centrum and the arch ; a ligament, however, often 

 passes from the neck of the rib to the neural arch. In the 

 anterior and posterior vertebrae they are often attached to 

 transverse processes. Ribs are found on all the precaudal 

 vertebrae except the atlas and sometimes the 

 next two, and on one or two of the presacral ver- 

 tebrae (lumbar). The proximal part alone ossifies, 

 the distal part remaining cartilaginous or be- 

 coming calcified. 



The posterior ribs present a peculiar modification 

 in Draco, being extremely long and serving to support 

 lateral expansions of the skin, by which the animal is 

 able to perform its nights. In the geckos and chamaeleons 

 the posterior ribs meet each other in the ventral middle 

 line forming complete hoops. 



Except in the Amphisbaenidae and some other 

 lizards in which the fore-limbs are absent, a variable 

 number of the anterior thoracic ribs are connected 

 by a cartilaginous sternal portion with the ster- 

 num. 



In all lizards except some of the Amphisbaeni- 

 dae there is at least a trace of a sternum (though 

 as we have seen it is not always connected with the 

 ribs), which increases in size in forms in which 

 the anterior limbs are more developed. The 

 sternum typically consists of a rhomboidal plate 

 of cartilage, from the posterior part of which a 

 single or double prolongation is continued back- 

 wards into the wall of the abdomen. A few (two 

 to four) sternal ribs are attached to the rhomboidal plate and 

 the remainder (one to four) to the posterior continuations (Fig. 

 185). 



In the skull (Fig. 186) there is a cartilaginous interorbital sep- 

 tum (except in the Amphisbaenidae). The temporal region (exoc- 



* Boulenger, P.Z.S., 1888, p. 351. 



Fm. 184. Ster- 

 num and ster- 

 nal ribs of 

 Chamaeleo 

 (from Gegen- 

 baur). St 

 sternum ; Mst 

 posterior part 

 of sternum, 

 (metast e r n- 

 u m) ; Co 

 coracoid ; c,c' 

 ribs. 



