CHAPTER VI. 



SPECIAL CHARACTERS OF THE SACRAL AND CAUDAL 



VERTEBRA. 



Sacral Vertebra,- -The difficulties in defining the sacral 

 vertebras have been noticed at page 30. Their essential 

 character is best illustrated by tracing it up from the simple 

 condition it presents in the tailed Amphibians (as Menopoma). 

 In these animals a series of similar small straight ribs are 

 movably articulated to the ends of the transverse processes 

 of all the trunk vertebrae, 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 vertebras. 



The inferior ossification of the transverse processes of the 

 true sacral vertebrae in Mammals (see Fig. 6, p. 26) is clearly 

 of the same nature, though more rudimentary in character, 

 and coalescing at an earlier period with the remainder of the 

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



