CHAPTER VI. 



SPECIAL CHARACTERS OF THE SACRAL AND CAUDAL 

 VERTEBR.^. 



Sac7'al Vertebrcz. — The difficulties in defining the sacral ver- 

 tebrae have been noticed at p. 24. Their essential character 

 is best illustrated by tracing it up from the simple condition 

 it presents in the tailed Amphibians (as Menopoina). In these 

 animals a series of similar small straight ribs are moveably 

 articulated to the ends of the transverse processes of all 

 tlie trunk vertebras, which are not distinctly divisible into 

 separate regions. To the distal extremity of one of these 

 the ilium is attached. This vertebra with its rib thus 

 constitutes the " sacrum," and the ilium is clearly seen not 

 to be a " pleurapophysis," as it is sometimes called, or any 

 part of a vertebra, but a something distinct and superadded. 

 In the Crocodiles there are two vertebrae with strongly 

 developed rib-like bones connecting them to the ilium, and 

 remaining long only suturally united to their vertebra. 



The inferior ossification of the transverse processes of the 

 true sacral vertebrae in Mammals (see Fig. 6, p. 21) is clearly 

 of the same nature, though more rudimentary in character, 

 and coalescing at an earlier period with the remainder of the 

 vertebra. It is not yet known that it exists in all Mammals, 

 but this may be considered probable, as it is certainly found, 

 at least in the first sacral vertebra, in such dift'erent forms as 



