68 CAUDAL VERTEBR.-E. [chap. 



As the tail is the principal organ of locomotion in the 

 Cetacea, it is always very well developed, and consists of 

 numerous (from 18 to 30) vertebras. 



Chevron bones are always present, and of simple character, 

 though with long compressed median spines (see Fig. 29, 

 p. 65). They are mainly attached to the posterior extremity 

 of the vertebra immediately in front of them. 



The characters of the caudal vertebras in the various 

 animals of the order are tolerably uniform, the tail having 

 the same function in all. In the anterior part of the region 

 the bodies are very massive and cylindrical ; the arches have 

 high spmes, with metapophyses on their anterior .edges, and 

 the transverse processes are tolerably long, and directed 

 straight outwards. In passing backwards the arches and all 

 the processes gradually disappear, and the bodies become 

 much compressed, and elevated vertically. Suddenly a 

 change takes place (at the spot where the end of the 

 vertebral column becomes enclosed in the horizontal, 

 laterally extended cutaneous expansions, constituting the 

 " flukes " of the tail), and the vertebrae altogether alter 

 their characters, becoming much smaller, wide transversely 

 and depressed. There is always one vertebra which is 

 transitional in its character between these two forms. Most 

 of the caudal vertebra are perforated by a vertical canal on 

 each side, at first passing through the base of the transverse 

 process, but posteriorly through the body of the vertebra 

 itself This transmits an ascending branch of the caudal 

 artery. 



The SiRENiA have numerous, much depressed caudal 

 vertebra, with wide transverse processes, gradually dimin- 

 ishing in length from the commencement towards the 

 apex. They are thus very different from those of the 

 Cetacea. 



