THE SKELETON OF REPTILES 



31 



is proved by the various stages of its evolution in the reptiles, 

 from the earliest (Fig. 15) in which it scarcely differs from 

 rhachitomous — as this structure is called — vertebrae of an early 

 amphibian, to the modern in which the structure is nearly like 

 that of mammals. 



In front of the atlas, that is, between it and the skull, there 

 was, in all early reptiles, as well as in some later ones, like the 

 crocodiles and tuatera, the remnant of 

 what is believed to have been another 

 vertebra, of which only the arch re- 

 mains, and which is called the proatlas. 

 In its earliest condition it articulated 

 with the skull in front and the arch of 

 the atlas behind. 



As in mammals, the vertebrae of- the 

 different regions have received distinc- 

 tive names, cervical, dorsal, lumbar, 

 sacral, and caudal. The numbers of 

 each region are far more variable than 

 they are among mammals, the total 

 number of vertebrae in the column 

 varying from about thirty to more than 

 five hundred, in certain snakes. Nor 

 are the different regions always easily 

 distinguishable, especially those in front 

 of the sacrum. In the earliest reptiles 

 there was practically no neck, and only 

 two vertebrae, the atlas and axis, that 

 properly can be called cervical. Very 

 soon, however, the reptiles developed a 

 longer neck with seven vertebrae, a 

 number that has remained singularly constant in higher animals, 

 especially in the mammals. In most modem reptiles there are 

 from seven to nine; in a few lizards, five. But the number was 

 much more inconstant among the older reptiles; some of the 

 plesiosaurs had as many as seventy-six cervical vertebrae; some of 

 the older lizards even had as many as eighteen. 



Fig. 14. — Rhachitomous 

 dorsal vertebra of Eryops: n, 

 neurocentrum or arch; pi, 

 pleurocentrum; i, inter- 

 centrum or hypocentrum; az, 

 anterior zygapophysis; pz, pos- 

 terior zygapophysis; .d, diapo- 

 physis, for tubercle of rib; p, 

 parapophysis, for head of rib. 



