248 ON THE INTERCENTRUM OF THE 



the pleurocentra as between their bases, a condition of things which is probably due 

 to disturbance of the parts before fossilization (Fig. 5). Fritsch also represents the 

 neural arch in Sphenosaui-us and Chelj^dosaurus as resting on the intercentrum, in 

 diagrammatic manner,''' and the pleurocentra and hypocentrum pleurale as alternating 

 with them. As Dr. Fritsch does not give profile figures of the original specimens, 

 one cannot judge how much may be due here also to disturbance of the parts. Still 

 more open to this question are the figures of Von Meyer.f The true relation of the 

 parts cannot be learned from these figures and descriptions, ^nd it is probable that the 

 specimens at the disposal of European paleontologists generally are as yet mucli 

 inferior in the condition of their perservation and in size to those found in Texas. 



The vertebrjB of the true Stegocephali are, so far as known, undivided and with- 

 out intercentra, with one exception, and that is the genus Branchiosaurus Fritsch. J 

 According to this author, each vertebral body is composed of thi'ce parts, a median, 

 an anterior, and a posterior. The median, if separated from the other two parts, would 

 be bi-concave, and it supports the rib. Each of the extremital segments presents a 

 convexity towards the median segment, resembling the epiphysis of a mammal. What 

 the homologues of the three jmrts may be I'craains to be ascertained. 



II. — THE KEPTILIAN INTERCENTRUM. 



The Rhynchocephalian genus Si)licnodon has been represented by Giinther to 

 have cei'vical and some dorsal intercentra.§ It therefoi'e seemed probable that its 

 structure might throw some light on the homologies of the batrachian and reptilian 

 intercentra. I therefore, dissected the vertebi-al column of a specimen in alcohol kindly 

 l^resented to me by my friend, Dr. James Hector, chief of the Geological iSurvey of 

 New Zeiiland. The attachment of the chevi'on bones does not, in the figui-e of Giin- 

 ther, resemble very closely that seen in Clepsydrops, but on examination I found the 

 relations to be structurally and homologically identical with that found in Crico- 

 tus. The caudal vertebne are se[)arated by a rather thick disk of intervertebral cai'ti- 

 lagc Avhich sun'ounds the foramen chordi\3 dorsalis, resembling closely the intercentrum 

 of the Embolomeri. There is an internal annular deposit of phosphate of lime, which, 

 if extended to the entire cartilage, would give us an embolomerous intercentrum like 

 tliat of Cricotus. This resemblance is a true homology, for the chevi'on bones spring 

 from this cartilage, although in sliglit contact with the centrum in front of it (Plate 

 I, Fig. 10). 



* Fauna der Goskolile, pp. 25-28. 



f Palnonlographica, 1858, Vol. vi. 



X Fauua dur Uaskolilc, 150. i, Heft. \. 



§ Transactions of the Royal Society Loudon, 18C7, iil. ii, fig. 23. 



