252 ox TIIK rXTERCEXTimM OF THE 



cosauria. But on this point we have as yet no evidence. On the other hand in the 

 rhaehitomous Ei-yops, where the centrum is not completed, we have seen that in the 

 cervical region the hypocentrum pleurale probably unites with the intercentrum in 

 front of it. Let us sui)pose this to have been the case throughout the vertebral 

 column. The interpretations would be as follows : 



In the caudal region of Eryops the hypocentrum i^leurale Avould bear the chevron 

 bone, and the present intercentrum would become the centrum. We would then have 

 in the caudal region of Archegosaurus and Cricotus, the remarkable state of aifairs of 

 two ti'ue centra present, one of which has an intercentrum coossified with it, and the 

 other supports the neural arch. The same would be the interpretation of the dorsal 

 structure in Cricotus. As there is no direct evidence of such structure in the vei'tebral 

 regions mentioned, noi" in the caudal region of the Pelycosauria, its assumption appears 

 to me to be the least probable alternative before us. Under this interpretation we have 

 to regard the pleurocentrum as either vanishing, or becoming coossified with the inter- 

 centrum (m., h^fpocentrum) in the vertebral centra of existing Batrachia, Avhich would 

 then consist of four elements each, viz., two pleurocentra, hypocentrum pleurale, and 

 intercentrum (hypocentrum), in the caudal i-egion at least. Of this no evidence can be 

 obtained from embryos of a length of 25mm and over. In Reptilia the centrum Avould 

 consist of pleurocentrum and intercentrum (hyi)ocentrum) combined, a division of which 

 we have no evidence in the emlirj'ology of the vertebral column in these animals. In 

 the divided caudal centra of the lizards one might see intercentrum (hypocentrum) and 

 pleurocentrum, but this supposition is not necessary to account for this structure. I 

 have observed above, that the evidence furnished by the vertebrse themselves is in 

 favor of this division having arisen in the middle of a true centrum. The chief objec- 

 tion to this interpretation of the reptilian centrum is, however, to be found in the 

 column of Cricotus, where we find the large bodies which bear the chevron bones, in 

 the process of extinction, leaving behind bodies which are homologous with the centra 

 of the Pclycosaurian reptile. And this is equally true, whether we regard the hypo- 

 centrum or the hypoeenti'um pleurale as the intercentrum, for it is the pleurocentra 

 which remain, to be the true centi'a of Cricotus. 



Thus on either of the assumj>tions just mentioned, the development of the dorsal 

 l)urt of the vertebral column in Cricotus is in an opposite direction to that stated by 

 Fiitsch to characterize the Sphenosauridai. This is the main point to be proven, li' 

 further I have shown that the larger dorsal bodies of Cricotus are homologous with 

 the centra of the Pelycosauria and Lacertilia, the i)roposition remains proven that the 

 inferior vertebral bodies of the Jihachitoiui and the entire vertebral bodies of existing 

 Batrachia, are intercentra and not centra. 



