THE VERTEBRAE 99 



are known only among amphibians. Certain ancient fishes {Eury- 

 cormus), it is true, with dorsal embolomerous vertebrae, have in the 

 tail pseudo-rhachitomous vertebrae, composed of two half-disks, the 

 one with its base below, the intercentrum, the other with it above, 

 the undivided pleurocentrum. 



The evolution, then, of the holospondylous reptilian vertebra from 

 the temnospondylous amphibian vertebra seems clear : by the simple 

 increase in size of the notochordal centrum and the progressive de- 

 crease of the intercentrum to a wedge-shaped, subvertebral bone, 

 and its final loss everywhere in the column save in the atlas and 

 chevrons of the tail ; and thus the term hypocentrum becomes purely 

 a synonym of the earlier term intercentrum. The retrogression of 

 the disk-like pleurocentrum into the paired pleurocentra of the 

 Rhachitomi, is paralleled by the separation of the primitively single 

 intercentrum into pairs, observed in Procolophon, many turtles, and 

 some plesiosaurs. 



Cervical Vertebrae 



(Figs. 77-81) 



The number of vertebrae in the neck or cervical region of reptiles 

 is not always easily determinable. In those reptiles having a sternum, 

 the first rib attached to it definitely determines the beginning of the 

 thorax. The distinction is almost as definite in those in which there 

 is a change in the articulation of the rib from the centrum to the 

 arch, as in the Sauropterygia and Archaeosauria. But the early 

 reptiles had no sternum, and free ribs were continuous from the atlas 

 to the sacrum without change in their mode of articulation. In such, 

 the changes in their shapes, with other modifications, may indicate 

 approximately the beginning of the dorsal series. Better evidence, 

 however, is found in the position of the pectoral girdle as found in 

 the rocks. 



The number is very variable, more so than that of the dorsal verte- 

 brae. The Cotylosauria, like the Temnospondyli, have but one or 

 two vertebrae which may properly be called cervical, since the pec- 

 toral girdle is almost invariably found lying immediately back of the 

 skull, the front end of the interclavicle, indeed, between the angle of 

 the jaws. Primitive reptiles, then, like their immediate ancestors, 

 the Stegocephalia, had practically no neck, and but little motion of 

 the head in a lateral direction. 



