89O SUB-KINGDOM VERTEBRATA. 



Leaving then the origin of Vertebrates as an unsolved problem, 

 in the solution of which the palaeontologist can have but little share, 

 we may proceed to a brief survey of the classification and chief 

 structural features of the Vertebrate sub-kingdom, so far as they con- 

 cern the palaeontologist. 



For this purpose the sub -kingdom may be divided into five 

 classes 1 — viz., Pisces or Fishes, Amphibia or Amphibians, Reptilia 

 or Reptiles, Aves or Birds, and Mammalia or Mammals — of which 

 some of the more important features, from the point of view of the 

 palaeontologist, will be mentioned under these respective heads. 



The first and second classes have been brigaded together by Pro- 

 fessor Huxley under the name of Ichthyopsida, and the third and 

 fourth as Sauropsida ; and these terms will frequently be found 

 convenient. Other writers, again, from the absence or presence of 

 certain structures during the course of development, group together 

 the two first classes as Anamniota (Anallantoidea, or Branchiata), 

 and the remaining three as Amniota (Allantoidea, or Abranchiata). 



Since the hard parts of Vertebrates are those with which alone 

 the student of Palaeontology usually has to deal, it will generally be 

 unnecessary in this work to make any allusion to the soft parts of 

 the body. With, regard, however, to these hard portions, it is ad- 

 visable to give an extremely brief sketch of the more important 

 elements of the Vertebrate endo- and exoskeleton for the benefit of 

 those readers who are unacquainted with the elements of Compara- 

 tive Osteology. It must, however, be distinctly understood by all 

 who desire to practically study the history and structure of extinct 

 Vertebrates, that it is absolutely essential they should have that 

 thorough knowledge of the osteology of the recent members of the 

 sub-kingdom which can only be gained by familiarity with actual 

 specimens, accompanied by patient and laborious study of the 

 numerous works on the subject. The following sketch is, indeed, 

 merely sufficient to enable the reader to understand the meaning of 

 the terms employed in the sequel ; and throughout the Vertebrata 

 the limits of this work will necessarily permit of only some of the 

 more salient features of the skeleton of the various groups being 

 mentioned. It may also be observed that those groups which have 

 wholly disappeared, or of which the palaeontological and evolutionary 

 history is of especial interest, are more fully treated of than those 

 more or less exclusively confined to the later or present epochs, and 

 of which the history is fully recorded in those works to which the 

 term " Natural History " is usually restricted. In the majority of 

 instances, again, space does not permit of allusion to species ; but 



1 A sixth class — Leptocardii — is formed for the reception of the Lancelet 

 {Amp/n'oxus), with which the palaeontologist has, unfortunately, no concern, as 

 its past history is a blank. 



