TERRESTRIAL. VERTEBRATA. 



249 



There can then be no donbt of the homolof^y of the cartilaginous interceutrnm of 

 Sphenodon with those of the Pc!3C0sauna, and of the entire centrum of Sphenodon 

 with that of the I'elj'cosauria. I'he division of many of the caudal centra of Sphe- 

 nodon and ol" Diany lizards, is evidently' not exi)laincd by the supposition that one of 

 the parts is an intercentiiini. That they aic halves of a single centrum is not only 

 rendered probable by the above determination of the intercentrum, but is supported 

 by the nKidifieations presented by the centra themselves. In Sphenodon, instead of 

 gradually losing one of the halves, the latter become, without diminution, more and 

 more consolidated towards the anterior part of the caudal series, and merge into the 

 ordinary t^'pe of vertebra, each proximal centrum representing both halves of a distal 

 one (Plate I, Fig. 10). 



It is thus probable that each vertebra of a Lacertilian* and of a Pelycosaurian 

 represents one centrum, and that intercentra are present in some types, and absent in 

 others, except as always represented by the chevron bones of the tail. Is this the 

 case with the existing: Batrachia ? 



Fio. ^.—Eryoiu megaccpJuilm. Vertebral column represented in Fig. 1, from below; oue-fourtli natural size. 

 From Permian epoch of Te.\as. 



In an important memoir, Iloffmanf presents us with the results of his investiga- 

 tions into the homologies of tlic ril)s of the terrestrial Vertcbrata. He finds that in all 

 of them, excepting the Batrachia, the primitive ribs are processes of the intervertebral, 

 and not of the vertebral caitilage. The intervertebral cartilage in reptiles with ball 

 and socket joints divides, each half uniting with its adjacent body, the one to form 

 the ball, and the other the concave extremity of a vertebra. In the Mammalia the 

 halves foi-m the epiphyses respectively. In this class the head of the rib justifies this 

 origin, lomaining as it does articulated in a fossa which is ci|ually excavated from two 

 adjacent epiphyses. I have shown thai the head of tliu lil' in thr Pclycosauria is artic- 

 ulated with the undivided intercentrum. t The striictine in tiiis order is in conlirnia 

 tion of the doctrine that the elcracni 1 have called intercentrum is such in fact. The 

 passage of time has seen in the Keptilia in general, the same moditication in the mode 

 of attachment of ribs as occurs in the vertebral column of the Mammalia, etc., in i>ass- 



» Baur 3how3 that in ihr Lncerlilian genera IJniplatc.s and Gecko, intercentra are present tliroughoul the dorsal, 

 lumbar, and sacral regions. American Naturalist, 188fi, p. 174. 

 t Niederl.-ondisches Archiv. f. Zoologic, 1878, p. 190. 

 X Proceeds. Amcr. Philos. Society, 1884, p. 43 ; Proceeds. Amer. Assoc. Adv.' Science, 1881, p. 471. 



