30 



WATER REPTILES OF THE PAST AXD PRESENT 



notochord more or less continuous, like a string of beads, the beads 

 representing the enlargements between the contiguous vertebrae. 

 In many early amphibians, and probably in all the earliest ones, 

 as well as in the fishes from which they were derived, the vertebra 

 is more complicated in that it is composed of at least three pairs 

 of^eparate bones, two of which united with each other, the third 

 finally disappearing in modern animals, or at thejmosjt represented 

 by a mere vestige called the intercentrum. The dorsal pair of 

 these bones, called the ncurocentra, forms the arch of the vertebra. 

 The ventral posterior pair, called the pleurocentra, increases in 



Fig. 13.— Notochordal cervical vertebrae, with intercentra, of Opiiiacodoii, a 

 primitive thcromorph reptile from the Permocarbonifcrous of New Mexico: pa. pro- 

 atlas; AH, arch of atlas; 0, odontoid; u.v, axis. 



size and unites to form the centrum or body of the vertebra; while 

 the ventral anterior pair, early united with each other, is called the 

 hypocentrum or intercentrum, persistent in all early reptiles as 

 a vestige between the centra on the ventral side. This divided 

 condition of the vertebra is persistent in the first_vertebra, the 

 atlas of all higher animals, in which the so-called body is the 

 hypocentrum or intercentrum, the arch is the neurocentrum, 

 while the pleurocentra have fused more or less with the anterior 

 part of the next vertebra, the axis, to form the so-called odontoid. 

 That this is the real explanation of the structure of the atlas 



