RHACHITOMOUS VERTEBRAE 599 



require but little change in the size of the different parts to develop 

 the second vertebra shown in B into one of those shown in C. In 

 any event I think the specimens show conclusively that the hypo- 

 centra or intercentra are not the hypocentra pleuralia, as Gadow 

 believes, nor the pleurocentra the fused pleurocentra and hypo- 

 centra, as Jaekel, Broili, and others believe. 



Cope suggested that the pleurocentra were eliminated in the 

 evolution of the holospondylous amphibian vertebra, but it seems 

 more reasonable to me that there has been a fusion of all three 

 elements in the Branchiosauria, Lepospondyli, and modern amphib- 

 ians, from the fact that none of these amphibians are known to 

 possess any vestiges as separate elements. 



If this theory be true, that is the union of the hypocentra with 

 some or all the other elements of the vertebrae in the amphibia, 

 and their final loss, save as simple intercentra and chevrons in the 

 amniota, it would offer of course the best class distinction between 

 holospondylous amphibians and reptiles. In any event the struc- 

 tural differences seem too radical to unite forms with free chevrons 

 articulating intercentrally with those having no free intercentra and 

 the chevrons exogenous processes from the body of the vertebra. 

 Nevertheless that is what is done in the order Microsauria. Baur 

 some years ago reached the conclusion that Hylonomus and Petro- 

 hates were undoubted reptiles,^ and his views were accepted by 

 Fiirbringer and others. A study of the specimen described by Cope 

 and doubtfully referred by him to the species Tuditanus punc- 

 tulatus Cope under the name Isodectes {Eosauravus copei Will., 

 Isodectes punctulatus Moodie, nee Cope) convinces me that the 

 genus is allied to Hylonomus, and consequently is a true microsaur, 

 since Hylonomus Dawson is the type, with Dendrerpeton Owen, and 

 Hylerpeton Owen, of the order Microsauria, as proposed by Dawson 

 in 1863 (Airbreathers of the Coal Period). Whether Hylonomus, 

 Petrohates, Eosauravus, and Sauravus Thevenin are true reptiles, 

 even though having free chevrons, will not be positively deter- 

 mined until the structure of the skull has been made out. What- 

 ever is the final disposition of them, they must be excluded from the 

 Amphibia, and doubtless the ordinal name Microsauria will remayi 



I Anatomischer Anzeiger, XIV. 



