244 



ON THE IXTERCEKTRUM OF THE 



properly termed an intcrcentrnm, so it is named by Gaudry the hypocentrnm. The 

 question as to which of these interpretations is correct has an important bearing on 

 the homologies of the corresponding parts in other batrachians and in reptiles ; and 



Fir.. 2. — EryopK mer/acephalui ; vertebral column from the left side, one-fdurth natural size. Original ; from 

 Proceedings American Philosnpli. Society, 1881. 



the phylogenies of these classes cannot be determined until the question is settled. 

 The following pages are devoted to this su])ject. 



I. — THE BATRACHIAN IXTERCENTRUM. 



That the intercentrum exists is shown by the very frequent occurrence in the 

 Pelycosaurian reptiles of the Permian epoch, of a wedge-shaped bone between the 

 vertebral centra on their inferioi- side* (Plate I, Fig. 9). Apparently homologous 

 elements occur in the dorsal and cervical regions of Sphenodon,t and in the cervical 

 regions of various other lizards. Similar pieces are found in the dorsal and caudal 

 regions of various Mammalia, for instance, Erinaceus.J But in general they are 

 Avanting from Mammalia, and are better developed in the Pelycosauria than in any 

 other order of reptiles. 



In the Pelycosauria (Clepsydrops, Dimetrodon), the intercentra of the caudal region 

 are continuous with, or form a wedge-shaped common head, of the chevron bones 

 (Plate I, Fig. 9). In the rhachitomous Eryops, in the caudal part of the column, 

 the i)ieces which correspond with the intercentra of the Pelycosauria in forming the 

 expanded heads of the chevron bones, are those which I have termed intercentra, and 

 which Professor Gaudry has called hypocentra (Plate I, Fig. 1). Here, as in the 

 dorsal region (Cut 2), the intercentra present their lateral angles upwards towards 

 the neural arch. The nciu-al arch rests exclusively on the pleuroeentrum, which in 

 turn adheres to the intercentrum behind it by its long side, and to that in front of it 

 by its short side or end. The diapophysis belongs exclusively to the basal part of the 

 neural arch in' the doisal and cervical regions, and its extremity forms a vertical 



*Cope, Proceeds. Amer. Pliilos. Soc, 1878, p. 510; 1880, p. 38. 

 t Giinlher, Transac. Royal Society, 1867, PI. ii, Fig. 17 ; Albrecht, 

 |: Meyer, Neues Jahrbuch f. Mineral., Hd. II, pp. 229-30, 



