GENERAL CHARACTERS. 897 



splints overlying the arch of the atlas in Crocodiles represent a 

 vertebra intercalated between the latter and the cranium, for which 

 the name p7'oatlas has been proposed. It is, however, by no means 

 proved that these splints do not belong to the atlas vertebra, so 

 that an authority like Mr Hulke is disposed to doubt the validity of 

 this alleged additional vertebral segment. 



In such Vertebrates as have the hind limbs well developed, one 

 or more of the vertebrae in the hinder part of the trunk differs from 

 the rest, and forms a sacrum (fig. 818, s), which articulates by 

 anchylosed ribs with the pelvic girdle. The vertebrae forming the 

 region between the sacrum and the last cervical vertebrae in the 

 higher forms consist posteriorly of a lumbar series (fig. 818, /), 



Fig. 818. — Skeleton of an Armadillo, showing the regions of the vertebral column, c, Cervi- 

 cal ; d, Dorsal ; /, Lumbar ; s, Sacral ; t, Caudal region or tail. The pectoral girdle is below d, 

 and the pelvic below s. Reduced. 



without or with very short ribs, and anteriorly of a dorsal series 

 (fig. 818, d), provided with long ribs, of which the anterior ones 

 articulate with the breast-bone or sternum. In many Amphibians, 

 however, in which (as in many Reptiles) there is no sternum, the 

 ribs continue throughout the lumbar and some of the caudal series, 

 as in fig. 814, so that no distinction can be drawn between dorsals 

 and lumbars, and the series between the cervicals and sacrals is 

 consequently called dorso-lumbar. The term caudal is applied to 

 all those vertebrae posterior to the sacrum. Moreover, where there 

 is no sternum, it is frequently difficult to draw an exact demarcation 

 between cervical and dorsal vertebrae, and we are compelled to rely 

 on analogies drawn from the mode of articulation of the ribs. 



Ribs, which have been already mentioned, are nearly always 

 present in the dorsal region, and may occur throughout the length 

 of the vertebral column. According to the views of Dr Baur, they 

 are to be regarded as originally intervertebral structures, since they 

 are attached in some of the earlier forms to the vertebral inter- 



