Apr. 1903. North American Pi.esiosaurs — Willistok. 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- 

 brae. The spine is incomplete posteriorly, but seems to have been 

 short, stout and much inclined. 



The structure of the plesiosaurian atlas and axis has be( n 

 described by Owen,* Huxleyt and Barratt.^ In the specimen of a 

 Plesiosaurus described by Barratt the different elements were sep- 

 arati d and were for the most part complete. The neurapophyses 

 difiet 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 Plesiosaurus etkeridgii, 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 Plesiosaurus 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 is more special- 

 ized than in these species of Plesiosaurus. 



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.^[ 

 It is a little interesting to note, however, that Owen (1. 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 Vertebrate, would be the 

 haemapophyses of that vertebra; and the odontoid process the true 

 centrum. " He concludes, 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. Man. Nat. Hist. xx. 217, 1850. 

 t Geological Journal, rf 



}.\nn. Man. Nat. Hist. Nov. iS;S. . 



5 American Naturalist. 1887. Sept., p. 830, 



ii Bull. Mus. Roy. d'hist. Nat. de Belg. ii. 185. 



• Mem. Amer. Mas. Nat. Hist. i. p. 1S7. 



