310 PHILOSOPHY OF ZOOLOGY. 



the spine by a single tubercle placed below the foramen. 

 The motion of the head is very limited in every direction. 

 In some of the cartilaginous fishes, the head is joined to 

 the vertebral column by two condyles ; but this articulation 

 is equally incapable of extensive motion as the former. 



The vertebral column is either cylindrical, angular, or 

 compressed. The vertebrae may readily be distinguished 

 from those belonging to the higher classes, by the peculiar 

 form which they exhibit The body of each is of a cylin- 

 drical figure, with a funnel-shaped depression at each end 

 It consists of concentric rings, which are supposed by some 

 to increase in number with the age of the animal. The 

 vertebrae are destitute of articular processes, and, when in 

 union, form, throughout the whole column, cavities compos- 

 ed of two cones, joined at the base. These cones contain a 

 cartilaginous substance formed of concentric fibres, of which 

 those next the tentre are the softest. By means of this 

 cartilage the vertebras are united, and upon it they per- 

 form all their movements. In the cartilaginous fishes all 

 the vertebras are consolidated together, so that the spinous 

 processes can only be distinguished. 



The vertebrae may be divided into the cervical, dorsal, 

 and caudal. In osseous fishes, the cervical vertebra are in 

 general wanting, although in some cases they exist, as in 

 the herring, to the number of four. In the cartilaginous 

 kinds, they are ossified into one piece. The dorsal verte- 

 bra are easily recognised, by wanting processes on the in- 

 ferior part. These have generally on the sides transverse 

 processes, to which the ribs are attached. The caudal ver- 

 tebra are possessed of spinous processes, both on the superi- 

 or and inferior surfaces. In those fish which are flat these 

 are very long, as in the flounders. The first caudal ver- 

 tebra is in general of a peculiar shape. The cavity of the 

 /rank is terminated by its central process. In the flotm- 



