584 REPTILIA [CH. 



persists in an undivided condition between throughout life. The 

 ventral portion of these cartilages, however, are ossified and appear 

 as the subvertebral wedge bones and chevrons, beneath the inter- 

 spaces between the vertebrae throughout the neck, trunk and tail, 

 and not as in Lizards in the neck and tail regions only. 



The ribs have three divisions, there being a small intermediate 

 piece intercalated between the dorsal and sternal rib. From the 

 dorsal rib a hook-like outgrowth, the uncinate process, projects 

 backwards, which overlaps the next rib as in Crocodiles and Birds. 



Behind the sternum there is a long series of rod-like bones, the 

 so-called abdominal ribs, embedded in the muscles of the belly. 

 They are placed parallel to the direction of the sternal ribs, that is, 

 they slope obliquely forwards and inwards. They are regarded as 

 membrane bones and supposed to correspond to the ventral bony 

 scales of th6 Stegocephala ; they are probably of older origin than 

 the osteoderms which underly the scales in many Reptiles and are 

 not directly homologous with them. 



All these peculiarities of the skeleton are found in many of the 

 oldest fossil reptiles. 



There is no proper copulatory organ : the cloaca is used for this 

 purpose as in the Urodela. 



Numerous fossil Reptiles with skulls quite indistinguishable 

 from that of Sphenodon, or differing from it only in unimportant 

 particulars, are found in the Triassic Rocks. They are known as 

 Di apsid (Gr. Si's, doubly ; at/as, an arch) Reptiles because, as already 

 explained, the remnants of the once continuous covering of mem- 

 brane bones persist as two temporal arches, or arcades, an upper 

 postfronto-squamosal and a lower jugal-quadratojugal arcade. 

 From such primitive Diapsid Reptiles modern Lizards, Snakes 

 and Crocodiles, as well as Sphenodon, are descended. The modern 

 descendants of this primitive stock have diverged in two directions. 



On the one hand, the original stock gave rise to descendants 

 with long flexible bodies and extensible jaws this latter feature 

 involving of course a movable quadrate. The cloacal opening 

 became converted into a transverse slit and copulatory organs, 

 became developed behind it. This stock includes the Snakes and 

 Lizards which are often included in the one comprehensive order 

 the Sauria. 



On the other hand, the descendants of the common ancestral 

 form diverged in the direction of heavily armoured forms, in which 

 membrane bones underlying the scales were developed and in which 



