THE VERTEBRAE 97 



sylvanian, and Lower Permian, but best in Cricotus (Fig. 76 a-c) 

 from Illinois and Texas. Rhachitomous vertebrae are much more 

 widely known in numerous forms from the Pennsylvanian and 

 Permian of various parts of the world. 



An embolomerous vertebra is composed of two subequal, noto- 

 chordal disks, the anterior one the intercentrum, or hypocentrum, 

 bearing the exogenous chevron in the tail; the posterior one the 

 pleurocenlrum; and the arch or neurocentrum, resting upon both the 

 intercentrum and pleurocentrum, but chiefly the latter. The artic- 

 ular surface for the head or capitulum of the ribs is chiefly on the 

 intercentrum; the surface for the articulation of the tubercle of the 

 rib, on either the arch or diapophysis. 



A rhachitomous vertebra (Fig. 76 d) differs in that the intercen- 

 trum or hypocentrum is more or less wedge-shaped, with its base on 

 the ventral line, its apex not reaching the dorsal side; while the 

 pleurocentra behind are paired, with the basal side above and their 

 apices reaching the ventral line only narrowly or not at all. The 

 neurocentrum, as in the embolomerous forms, is borne by all three 

 bones, but chiefly by the pleurocentra. The head of the ribs articu- 

 lates with the intercentrum, the tubercle with the diapophysis of the 

 neurocentrum. 



The earliest known amphibian vertebrae are embolomerous; rha- 

 chitomous and holospondylous vertebrae appearing later, so far 

 as our present knowledge goes. And this is one of the reasons why it 

 would seem that the embolomerous type is the more primitive, giv- 

 ing origin directly to the reptilian holospondylous type, as was first 

 suggested by Cope ; that the rhachitomous type was derived from it 

 by the loss of the upper part of the intercentrum and the lower part 

 of the pleurocentrum and the division of the latter into two lateral 

 parts. This reversion of the pleurocentrum to a more primitive onto- 

 genetic condition is the chief objection to this theory, nevertheless it 

 is the more probable. We have seen that the more primitive phylo- 

 genetic condition of the intercentra persists longest in the neck and 

 tail. In the caudal vertebrae of Eryops (Fig. 76 d), and probably 

 other rhachitomous amphibians, there is an intermediate condition 

 between the embolomerous and rhachitomous types, in which the 

 single pleurocentrum is typically embolomerous, that is, disk-like 

 and perforated for the notochord; while the intercentrum bearing 



