978 The American Naturalist. [November, 
intercentra of the Rachitomi are not centra, but true intercentra, as 
everybody has believed so far. 
We only need to examine some of the Stegocephalia in which the 
ossification of these elements is more advanced. In old specimens of 
E megacephalus Cope, the pleurocentra are closely united to the 
posterior hase of the neural arches; the intercentra are placed between 
the pleurocentra below and do not reach the neural arches. Only the 
first intercentrum is connected with the neural arches of the first verte- 
bra the atlas, forming an atlas-ring as in all Amniota. How is it pos- 
sible that this first intercentrum can be a centrum? ‘The pleurocentra 
of Eryops are homologous to the centra of the Amniota. The rachito- 
mous condition is the most primitive one. Before Jackel, I described 
the condition in Archegosaurus as follows:® If we examine the 
vertebral column of Archegosaurus, we see that the notochord is still 
developed and that in the dorsal region each body of the vertebra 
consists of three parts, two lateral ones, the pleurocentra, and one 
inferior one, the intereentrum (hypocentrum). In the tail region we 
find even five elements; the two pleurocentra and below them two 
small hemacentra Hay” (hypocentra pleuralia) and the intercentrum 
to which the hemal arches (chevron bones) are attached. 
There is very little doubt, that in the caudal vertebra of the Rachi- 
tomi the hzmacentra, if they were present, formed with the pleurocentra 
a cartilaginous ring, In the precaudal region only the pleurocentra 
become ossified and support the neural arches. The intercentra were 
continued dorsad as cartilage, also forming a ring. In the tail the 
chevrons, lower arches, are united with them. This primitive condition 
is modified in two ways. First: the tntercentrum increases in size, 
especially the lower portions, become broader, until they meet with each 
other. The pleurocentra become reduced, or confluent with the upper 
arch. The intercentra form the body ofthe vertebre of the Labyrin- 
thodontia, they are wedge-shaped, the notochord never passes through 
the centre, but is placed in an excavation at the upper free border of 
the intercentrum, or there is a small fossa at the upper posterior face of 
the atlas, or there may be a chordal foramen just below the upper 
border. From all this it is evident, that the “ centra ” of the Labyrin- 
thodontia are intercentra. This opinion is now generally accepted, 
*Baor, G. TheStegocephali. A ip aaa Study. With 8 Fig. Anat Anz, 
XI, Bd., N. 22. Merz 20, 1896, p. 657-6 
Hay, O. P. On the structure and ee of the Vertebral column i 
Amia. Field Columbian Museum. Publication 5, Zoolog. Series. Vol. I, No. I, 
p- 40. Chicago, U. S. A. October, 1896. 
