70 



ANATOMY OF VERTEBRATES. 



3i 



V 



57 by some of the great reptiles, with 



limbs more adapted for terrestrial pro- 

 gression, called ' Dinosauria' ; favour- 

 ing in these the flexibility of the neck, 

 as the same ball-and-socket structure 

 does in the large herbivorous quadru- 

 peds of the present day. 1 The neural 

 arch in the dorsal region of Dinosauria, 

 was enlarged and strengthened by a 

 bony platform, with supporting ridges : 

 the sacrum included from four to six 

 vertebra?, having the neural arch 

 shifted so as to rest upon two cen- 

 trums and bind them together. 



§ 25. Vertebral column of Ptero- 

 sauria. — In tracing the modifications 

 of the skeleton from the earliest forms 

 of extinct species, the procoelian type of 

 A'ertebra appears first in the extinct 

 group of Reptiles (.Pterosam-ia) adap- 

 ted for flight ; the Pterodactyles of the 

 Lias show it, with a confluent neural 

 arch and a pneumatic foramen on each 

 side of the vertebra. 2 The cervical ver- 

 tebra? of Pterodactyles, fig. Ill, are the 

 largest, seven or eight in number, of 

 Avhich the first two coalesce. The atlas 

 has a very short discoid centrum and 

 two slender neurapophyses. The dorsal 

 vertebra? become smaller to the pelvis ; 

 they may be fifteen in number, fol- 

 lowed by two lumbar, from three to 

 seven sacral, and a variable number 

 of caudal vertebra?. One family of 

 Pterodactyles had a long and stiff tail ; 

 the rest, as in fig. Ill, a short tail. 

 The anterior free ribs have bifurcate 

 heads ; and, as this structure is asso- 

 ciated in modern Beptilia, with a 

 four-chambered heart, that organ had 

 probably reached the same stage of 

 perfection in the flying Reptiles, the 



1 clxiii. parts v. and vi. ; xxx. p. 285. 2 clii. p. 161, pi. x. 



$ 



Skeleton of Alligator 



