596 



5. W. WILLISTON 



in its posterior end, that in apposition with the body of the axis, 

 a cavity which extends through the bone as the notochordal canal. 

 Not only does this cavity extend through this bone but its orifice, 

 in Varanosaurus at least, is in apposition with a similar cavity in 

 the occipital condyle, conclusively proving the nature of the 

 basioccipital. Between the odontoid and the axis, below, there is 

 a large, massive intercentrum, even larger than the atlantal hypo- 

 centrum. This latter intercentrum gives support only in part to 

 the neuropophyses of the atlas, which rest chiefly on the odontoid. 



Fig. 3. — A, Atlas and axis of Dimetrodon incisivus Cope; B, caudal vertebrae of 

 undetermined amphibian; C, vertebrae of Desmospondylus anomalus Will.; N, neuro- 

 centra; H, hypocentra; P, pleurocentra. 



There can be no question that the odontoid is the combined 

 pleurocentra of the- atlantal vertebra, which has so far retained 

 its primitive character that it gives chief support to the atlantal 

 neuropophyses. Yet more conspicuously holospondylous in char- 

 acter is the atlas of P oecilos pondylus Case, in which the dorsal 

 arch appears to rest wholly upon the odontoid, and articulates 

 in the usual way with the axis.^ In Eryops, Cacops, and doubtless 

 all other rhachitomous genera the vertebrae of the trunk have a 

 perforating canal for the notochord formed by the junction of the 

 pleurocentra in the middle above the hypocentrum, the three bones 

 forming the canal; and it is chiefly because of this fact that Broili 



I Bulletin Amer. Miis. Nat. Hist. (1910). 



