CACOPS ASPIDEPHORUS 267 



views as to the fate of the pleurocentra will militate against the com- 

 monly accepted theory of the structure of the atlas, where the united 

 pleurocentra do not fuse with the hypocentrum, but remain distinct as 

 the odontoid process of the axis. But we have in such forms as Dimetro- 

 don, as well as all other Permian reptiles, to say nothing of the modern 

 lizards, a very large element, often larger than the atlantal hypocentrum 

 itself, intercalated between the atlas and the body of the axis below\ If 

 it be an intercentrum purely, it has assumed enormous proportions, sug- 

 gesting very forcibly the original size of the intercentra throughout the 

 column. If it be a hypocentrum like that of the atlas, we are irresistibly 

 driven to the conclusion that either the body of the axis is its pleuro- 

 centra, or else that the pleurocentra and neurocentra of an intercalating 

 \'(^rtebra have disappeared between the axis and atlas, leaving only the 

 large hypocentrum. And if this hypothesis be true, then a like explana- 

 tion would be necessary to account for all the intercentra, which certainly 

 are morphologically identical with the intercalating element in Dimetro- 

 flon. But such a theory is altogether too tenuous for me. It may be all 

 xQTj well to account for the ossifications between the centra in Sphenodon 

 a? new elements originating from one knows not what and call them 

 intercentra or subcentra, as Jaekel has very superfluously renamed them, 

 but will some one give a reasonable explanation of the very large preaxial 

 intercentrum in Dimetrodon and the other early reptiles, as well as the 

 lizards, on this h3rpothesis? 



Again, it is a remarkable fact, for which no reasonable explanation has 

 ever been given save by Cope, that in all true amphibians, both ancient 

 and modern, the chevrons are an integral, inseparable, exogenous part of 

 the body of the vertebrae, while in all reptiles and higher vertebrates they 

 are the intercentrum or part of it. In other words, they are in all 

 rhachitomous and embolomerous amphibians a part, of the hypocentrum, 

 a prolongation of its lower part, perforated for the passage of vessels, im- 

 movably united, while in all amniota they are freely articulated between 

 the vertebrae, or, as in some lacertilia, with the posterior part of the body 

 itself. Free chevrons have never been discovered in true amphibians, and 

 imtil they are we are compelled to assume that they are thehypocentra, 

 and thus, under the second hypothesis, must be morphologically distinct 

 from the chevrons of the amniota which are attached intercentrally. 



It is a remarkable fact that intercentra have never been discovered in 

 any amphibian, ancient or recent. In the Microsauria we have in some 

 cases well developed vertebrae associated with a fully ossified skeleton, but 

 so far no intercentra have been discovered in this group, so far as I am 

 aware, nor free chevrons, save in Tlylonomus. Again, the primitive rib 



