No. 397-] MOSASAURS, LIZARDS, AND SPHENODON. 3 
Dinosauria, Pterosauria, Chelonia, are regarded by some authors 
as vestiges of a “proatlas’’ or degenerate vertebra between the 
atlas and the skull. As remarked by Baur, these pieces correspond 
in position with the * neurapophyses ” of a typical vertebra. 
2. Atlas. The lateral pieces of the atlas proper are by all 
authors regarded as neural arches or ** neurapophyses ” (z.a. in 
our figures). 
3. Atlas. In his early papers Cope — in fact, when he first 
defined the *intercentrum " — regarded the ventral pieces of 
atlas and axis as “ intercentra " (7.7 and z.2 in our figures). The 
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Fic. 2.— Chelydosaurus vranii Fritsch. A rachitomous amphibian vertebra from the Permian 
of Bohemia, viewed from the righ? side, after Fritsch. Neurapophyses, neural arch 
2 pieces; pleurocentra, on sides of notochord, 2 pieces; hypocentrum pleurale, below 
notochord, 1 piece; intercentrum arcale, below notochord, 1 piece. 
anterior ventral piece (7.7), or lower element of the atlas ring, is, 
however, described as a “centrum” (z.¢., * pleurocentrum ") by 
Baur in his latest paper; by Gegenbaur as possibly an “ hypa- 
pophysis" (Vergleichende Anatomie, 1898, p. 249). In an 
unpublished lecture chart Baur rightly interprets ventral pieces 
of both atlas and axis as “intercentra.”’ 
4. Axis. The odontoid process is regarded as the pleuro- 
centrum or centrum proper of the atlas, which has become 
secondarily attached to the axis (oZ. in our figures). 
5. Axis. The posterior ventral piece (7.2 in our figures) is. 
described as an * intercentrum " by Baur, as the “atlantar hypa- 
pophysis " by Williston, in the Mosasaurs. 
