6 THE AMERICAN NATURALIST. [VoL. XXXIV. 
2. CONDITIONS IN THE MosASAURS, LIZARDS, AND SPHENODON. 
The modifications of the primitive type, in which each centrum 
has an intercentrum below and in front of it, are well illustrated 
in the accompanying diagrams (Fig. 4). 
The condition of the Platecarpus axis and atlas is even more 
primitive than that of Sphenodon ; but Sphenodon is more primi- 
tive than Platecarpus in the vertebrae behind the axis. 
A. In Platecarpus both atlas and axis intercentra free and 
wedge-shaped.  Intercentra 3-7 shifted forward secondarily 
upon the short hypapophyses of centra 2-6. 
D. In Sphenodon a *proatlas" ; intercentrum I of atlas 
loosely connected with neurapophysis of atlas ; intercentrum 2 
of axis completely coalesced with axis ; intercentra 3—7 in their 
- primitive position. ee 
In Varanus intercentrum 1 broadly connected with atlas 
neurapophysis ; intercentrum 2, forming an anterior hypapophysis 
upon axis, loosely connected in young, suturally united in adults ; 
intercentra 3+ forming tips of the long hypapophyses of centra 
2+. (This hypapophysial connection of the intercentra is an 
advance upon that initiated in Platecarpus.) 
C. In Cyclurus intercentrum I broadly united with atlas ring ; 
intercentrum 2 completely coalesced with axis ; intercentra 3 and 4 
in primitive position, but expanding to function as hypapophyses ; 
intercentra 5 and 6 small, in primitive position. 
The secondary modifications have, therefore, been of four 
kinds : : 
1. Conversion of the atlas intercentrum 1 into the basal piece 
of the atlas ring by loss of its wedge-shape and broadening of 
its contact with the neurapophyses (e.g., Varanus, Cyclurus). 
2. Lateral or complete union of the axis intercentrum 2 
with the anterior portion of the axis centrum (e.g., Cyclurus, 
Sphenodon). 
3. Secondary conversion of the intercentra 2+ into hypa- 
pophyses (e.g, axis of B, C, D; axis and third cervical of 
Cyclurus). j 
4. Shifting of the intercentra 3-7 forward upon the hypa- 
pophyses of the preceding vertebree (e.g., Varanus, Platecarpus). 
