Apr. 1903. NortH AMERICAN PLESIOSAURS—WILLISTON. 35 
a thickened, rugose, horizontal ridge, reaching to the intervertebral 
notch. The posterior zygapophyses are situated rather high, and do 
not differ materially from the same processes in the following verte- 
bre. The spine is incomplete posteriorly, but seems to have been 
short, stout and much inclined. 
The structure of the plesiosaurian atlas and axis has been 
described by Owen,* Huxley} and Barratt.{ In the specimen of a 
Plesiosaurus described by Barratt the different elements were sep- 
arated and were for the most part complete. The neurapophyses 
differ markedly in their expansion inward to form a roof for the neural 
canal, though they do not touch each other. ‘Theatlantal intercen- 
trum also differs in its posterior projection into ‘‘two long processes,”’ 
which are, however, broken away, leaving only their bases. The 
axial rib seems to articulate with the axis and axial intercentrum 
only. In VPlesiosaurus ctheridgit, the atlas and axis of which are 
described by Huxley, the bases of the atlantal neurapophyses are 
much larger and meet above the odontoid. In Plestosaurus pachyomus, 
as described by Owen, ‘‘the anchylosed bases of the neurapophyses 
form the upper border of the cup,” and the atlantal intercentrum 
‘develops a thick but short rough tuberosity from its under part,”’ 
and the rib projects from the centrum of the axis only. The pro- 
cesses were all broken away. 
It is seen that the structure in Dolichorhynchops 18 more special- 
ized than in these species of Plestosaurus. 
The atlantal and axial intercentra, variously considered by differ- 
ent authors as ‘‘subvertebral wedge-bones,”’ hypapophyses or hypo- 
centra, are correctly homologized by Baur,§ Albrecht|| and Osborn.4 
It is a little interesting to note, however, that Owen (Il. c.) long ago 
gave a correct hint of their homology: ‘‘ According to the latter view, 
what has usually been regarded as the centrum or body of the atlas 
in Saurians, Chelonians and the higher Vertebrata, would be the 
haemapophyses of that vertebra; and. the odontoid process the true 
centrum.” Heconcludes, however, that these elements are ‘‘ detached 
cortical parts of the real centrum;” though later he correctly compares 
them with the hypocentra of the labyrinthodonts. 
Perhaps the most primitive and unchanged condition of these ele- 
* Ann. Mag. Nat. Hist. xx, 217, 1850. 
+ Geological Journal, 1858. 
¢ Ann. Mag. Nat. Hist. Nov. 1858. sy 
§ American Naturalist, 1887, Sept., p. 839. 
|| Buil. Mus. Roy. d’hist. Nat. de Belg. ii, 185. 
§| Mem, Amer. Mus. Nat. Hist. i, p. 157. 
