XXIll] 



EXTINCT FORMS 



603 



VI 



form vertebrae, are represented by basidorsals, basiventrals, and 

 ventral intercalary pieces, the dorsal intercalary piece as in all 

 Reptilia being suppressed, while in the skull the basi-occipital 

 region is ossified. In the Sandstones lying above the Coal in- 

 dubitable Reptiles with 

 fully formed vertebrae 

 make their appearance. 

 Some of these, termed 

 the Cotylosauria, still re- 

 call the Stegocephala in 

 possessing a complete 

 roof of dermal bones 

 covering the skull. 



The descendants of 

 the Cotylosauria in the 

 next period split into two 

 stocks, in one of these 

 the Monapsida, the cover- 

 ing of dermal bones on 

 the head was reduced in 

 such a manner to leave 

 one broad temporal bar. 



The limbs in all these 

 early Reptiles were short 

 and stout, the fore- and 

 hind-limbs being nearly 

 of the same size. One 

 group of the Monapsida, 

 the Theromorpha, were 

 distinguished by having 

 the teeth differentiated 

 into incisors, canines and 

 molars and in having the 

 dentary very large and ar- 

 ticulating with the squa- 

 mosal, the other bones of 

 the lower jaw being re- 

 duced in size. These are 

 almost certainly the ancestors of Mammalia. 



In another group of Monapsida the teeth were reduced to a 

 pair of tusks in the upper jaw or were totally absent. This group 



FIG. 301. Diagram of arterial arches of 

 Crocodile viewed from the ventral aspect. 



i, n, in, iv, v, vi. First to sixth arterial 

 arches. 12. Tracheal (ventral carotid). 

 13. Common carotid (dorsal carotid) [right 

 side nearly atrophied]. 15. Eight sys- 

 temic arch. 16. Left systemic arch. 

 17. Dorsal aorta. 19. Pulmonary. 20. In- 

 nominate. 21. Subclavian (ventral type). 

 24. Coeliac. 



