RHACHITOMOUS VERTEBRAE 595 



If then we assume that the holospondylous vertebra has been 

 evolved from the rhachitomous, it is a matter of much interest to 

 determine how the evolution has occurred. The view which has 

 obtained general acceptance, among American paleontologists at 

 least, is that of Cope, so vigorously defended by Baur,"^ namely, 

 that the pleurocentra have progressively developed to form the 

 centrum of the amniote vertebra, the hypocentrum degenerating 

 into the vestige usually called the intercentrum; while, as pro- 

 posed by Cope and tentatively accepted by Baur, in the modern 

 amphibians it has been the hypocentrum which has developed into 

 the centrum, the pleurocentra disappearing. The theory more 

 generally accepted by European writers is that the pleurocentra 

 and hypocentrum have fused to form the centrum of all the higher 

 vertebrates, the small elements called the intercentra representing, 

 according to Gadow and others, the hypocentra pleuralia, which 

 have been rarely found in the tail of certain temnospondyls. Or, 

 in the words of Broili: 



Bei den Rhachitomen das Hypozentrum den ventralen Halbring und das 

 Paar der Pleurozentren den dorsalen Halbring des Wirbelkorpers reprasentirt ; 

 anderseits folgt daraus, dass weder das Hypozentrum noch die Pleurozentren 

 allein dem eigentlichen Wirbelkorper der Amnioten homolog sind, sondern das 

 beide zusammen Hypozentrum plus Pleurozentren desselben entsprechen.^ 



A study of the material in the University of Chicago collections 

 has convinced me of the general correctness of Cope's contentions 

 and the incorrectness of the opposing views. 



It is well known that in the older reptiles the odontoid of the 

 atlas is a larger bone than in modern reptiles or higher vertebrates, 

 and also that there is in the oldest forms invariably a large inter- 

 calating intercentrum between the odontoid and the body of the 

 axis below, a bone that is small or wanting in modern reptiles, as 

 also the older Crocodilia. In Dimetrodon, as will be seen in the 

 accompanying figure (Fig. 3), the odontoid functions as the real 

 centrum of the atlas, reaching quite to the ventral side between 

 the atlantal and axial hypocentra. It has a deep conical cavity 



1 "Everybody is convinced tliat the pleurocentra of the Rhachitomi represent 

 the centra of the higher vertebrates; and that the intercentra are homologous to the 

 intercentra of the Sphenodontidae," etc. — Amer. Nat. (1897), 975. 



2 Monatschr. d. deutschen geologischen Gesellschaft, LX (1908), 240. 



