72 EVOLUTION AND GENETICS 



small, paired bones, which may fuse to form a single bone. The 

 frontals also are ordinarily fused to form a single large bone, but 

 in a very small percentage of cases they remain separate. The 

 supraoccipital is usually fused with other elements in the occipital 

 bones, but sometimes persists as a separate bone between the 

 parietals and occipital, as is normally the case in some other 

 mammals. Both frontals and parietals are proportionally much 

 greater in size than in the lower animals because of the large size 

 of the brain, which they enclose. 



The Spinal Column. In the remainder of the axial skeleton 

 relationships are quite obvious. Vertebrae in all of the classes 

 above the Cyclostomes consist of a solid centrum from which a 

 dorsal neural arch arises, enclosing and protecting the spinal 

 cord (Fig. 47). This arch is surmounted by a spinous process 

 which furnishes attachments for muscles. Above the fishes the 

 neural arch bears two anterior and two posterior articular processes 

 which aid in preserving a firm articulation of successive vertebrae, 

 and a pair of transverse processes. The centrum may also bear 

 short transverse processes and a ventral arch, called the haemal 

 arch, which is well developed in the fishes and forms a conspicuous 

 appendage of some reptilian vertebrae. The haemal arch termi- 

 nates in a haemal spine. 



Ribs in many fishes are merely the halves of incomplete haemal 

 arches. In some fishes, amphibians and reptiles, ribs of this type 

 are found attached to the same vertebrae that bear other ribs, 

 more dorsal in position (Fig. 47e, f). The former are commonly 

 called fish ribs, and the latter true ribs. True ribs in their typical 

 form have two heads, one of which articulates with the lateral 

 process of the centrum, and the other with that of the neural 

 arch. 



The Sacrum. Terrestrial animals have the pelvic girdle 

 attached to the spinal column, and one or several vertebrae are 

 modified for this attachment (Fig. 48). In Necturus, a urodele 

 amphibian, only one vertebra is involved, usually the 19th, some- 

 times the 20th, and rarely the 18th, although obUque attachments 

 are on record in which the pelvic girdle joined the left side of 

 one of these vertebrae and the right side of another. Such verte- 

 brae are called sacral vertebrae, and when more than one is 

 involved a fusion often occurs, resulting in the development of 

 a composite bone, the sacrum. Wilder notes that "this anchylosis 



