No. 397.] MOSASAURS, LIZARDS, AND SPHENODON. 5 
A likelier rachitomous prototype is that afforded by Disco- 
saurus Credner, another Stegocephalian, in which it is seen 
(Fig. 3) that the intercentrum is in front of its corresponding 
pleurocentrum or centrum. Such a vertebra consists of five 
pieces. If from such an atlas prototype the pleurocentrum (#/.) 
were to be transformed into the odontoid process, the first result 
Fic. 4. — Typical cervical vertebrz of Mosasaurs, lizards, and Sphenodon. A. Platecarpus, a 
Mosasaur, with left neural arch of atlas removed. B. Varanus, a monitor lizard, with in- 
—* Lagoa assed rein to P of ne: E _Cyclurus a lizard, with — 
pting z 
with 1 hypapophysis « ads axis. ^P. "Sphenodon, a Riyocoepain w with all intercentra in 
axis, as in Cyclurus 
would be to bring the atlas and axis intercentra together with- 
out change of form. This is exactly what we find in Platecarpus, 
one of the Kansas Mosasaurs (Fig. 1). The photograph repre- 
sents a condition in which the intercentra 1 and 2 are alike in 
their wedge-shaped form, and are still entirely free from the other 
elements of the atlas and axis (see also Fig. 4). 
