300 VERTEBRATA. 



difficulty comprehend the terms which are necessarily em- 

 ployed in the definitions of the various groups. It may be 

 added here, before proceeding further, that it does not seem 

 requisite to treat the Vertebrata 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 frequently 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 palae- 

 ontologist, to give a general account of each class of the Verte- 

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

 leading forms of each. Only in cases of special interest will any 

 details of a more minute character than the above be given. 



The skeleton of the Vertebrata may be regarded as consisting 

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

 on the one hand (sometimes called the " axial " skeleton), 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 segments 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 segments which form the verte- 

 bral column are called " vertebras," and they have the follow- 

 ing general structure : Each vertebra (fig. 268, A) consists of 

 a central piece, which is the fundamental and essential element 

 of the vertebra, and is known as the " body " or " centrum" (c). 

 From the upper or posterior surface of the centrum spring two 

 bony arches (n ), which are called the " neural arches " or 

 " neurapophyses," because they form with the body a canal the 

 " neural canal " which encloses the spinal cord. From the 

 point where the neural arches meet behind, there is usually 

 developed a longer or shorter spine, which is termed the " spi- 

 nous process " or " neural spine " (s). From the neural arches 



