2 THE AMERICAN NATURALIST. [Vor. XXXIV. 
Mosasaurs, lizards, and Sphenodon, either in a primitive position 
or secondarily modified to subserve a variety of functions. 
The best known modification is in the atlas, where the inter- 
centrum forms the base of the ring. The modifications which 
have not been so generally recognized are in the axis and the 
other cervicals, where the intercentra secondarily function as 
hypapophyses, while still remaining more or less distinct from the 
centra proper. 
The primitive relations of the intercentra and pleurocentra 
(or centra proper) in the axis and atlas are beautifully shown in 
Fic. 1. — Platecarpus corypheus Cope, Coll. American Museum Natural History. Atlas, 1 
axis, 2; third cervical, 3. ight neural arch, z.a., in place; left neural arch rémoved, 
exposing odontoid aol A Sg of atlas), od. loosely articulated with axis. 
Intercentrum of atlas, 7.7; intercentrum of axis, 7.2; intercentrum of third cervical, 2.3; 
intercentrum of fourth peas 1.4. pet 3 and 4 are secondarily shifted forward 
upon the hypapophyses of the centra in fro 
the accompanying photograph (Fig. 1), taken from the neck of a 
specimen of Platecarpus — one of the Kansas Mosasaurs. But 
before describing this it will be well to recapitulate. 
The prevailing interpretations of the neck components in 
different types of reptiles are as follows: 
1. Proatlas. The pair of small dorso-lateral elements in 
Rhyncocephalia (see Fig. 4, D), certain Lacertilia, Crocodilia, 
