VERTEBEATA. 101 



terms which are necessarily employed in the definitions of 

 the various groups. It may be added here, before proceed- 

 ing further, that it does not seem requisite to treat the Fer- 

 tebrata with the same fulness as the Invertebrata. The 

 fossil remains of Vertebrates are in many cases of the 

 highest theoretical interest, but they come much less fre- 

 quently under the notice of the ordinary student than do 

 the remains of the Invertebrates. No practical study, also, of 

 the fossil Vertebrates can be carried out without a consider- 

 able acquaintance with Comparative Osteology. Lastly, the 

 remains of Vertebrate animals generally occur in such a frag- 

 mentary condition that a sufficient series of specimens for pro- 

 fitable study can rarely be obtained, except under peculiarly 

 favourable circumstances, in special cases, or where access can 

 be had to a first-rate museum. For these and other reasons 

 it is thought enough, in a treatise intended for the working 

 palaeontologist, to give a general account of each class of the 

 Vertebrata, with definitions of the orders, and a brief notice 

 of the leading forms of each. Only in cases of special inter- 

 est will any details of a more minute character than the 

 above be given. 



The shehton of the Vertebrata may be regarded as consist- 

 ing essentially of the bones which go to form the head and 

 trunk on the one hand (sometimes called the " axial " skele- 

 ton), and of those which form the supports for the limbs 

 (" appendicular " skeleton) on the other hand. The bones 

 of the head and trunk may be looked upon as essentially 

 composed of a series of bony rings or segments, arranged 

 longitudinally, one behind the other. Anteriorly these seg- 

 ments are much expanded, and likewise much modified, to 

 form the bony case which encloses the brain, and which is 

 termed the cranium or skull. Behind the head the segments 

 enclose a much smaller cavity, which is called the " neural " 

 or spinal canal, as it encloses the spinal cord ; and they are 

 arranged one behind the other, forming the vertebral column. 

 The seo-ments which form the vertebral column are called 

 " vertebrae," and they have the following general structure : 

 Each vertebra (fig. 480, a) consists of a central piece, which 

 is the fundamental and essential element of the vertebra, 



