LARGER GROUPS IN THE VERTEBRATA. 1 97 



animals (Cyclostoma), which includes the Hags (Myxinoida) 

 and Lampreys (Petromyzonta). The third class contains 

 only the genuine Fish (Pisces) : the Mud-fishes (Dipneusta) 

 are added to these as a fourth class, and form the transi- 

 tion from Fish to Amphibious animals. This distinction, 

 which, as will be seen immediately, is very important for the 

 genealogy of the Vertebrate animals, increases the original 

 number of Vertebrate classes from four to eight. 



In most recent times a ninth class of Vertebrata has been 

 added to these eight classes. Gegenbaur's recently published 

 investigations in comparative anatomy prove that the 

 remarkable class of Sea-dragons (Halisauria), which have 

 hitherto been included among Reptiles, must be considered 

 quite distinct from these, and as a separate class which 

 branched off from the Vertebrate stock, even before the 

 Amphibious animals. To it belong the celebrated large 

 Ichthyosauri and Plesiosauri of the oolitic and chalk periods, 

 and the older Simosauri of the Trias period, all of which are 

 more closely allied to Fish than to Amphibious animals. 



These nine classes of Vertebrate animals are, however, by 

 no means of the same genealogical value. Hence we must 

 divide them, as I have already shown in the Systematic 

 Survey on p. 133, into four distinct main-classes or tribes. In 

 the first place, the three highest classes. Mammals, Birds, and 

 Reptiles, may be comprised as a natural main-class under 

 the name of Amnion animals (Amnionata). The Amnion- 

 less animals (Anamnionata), naturally opposed to them as 

 a second main-class, include the four classes of Batrachians, 

 Sea-dragons, Mud-fish, and Fishes. The seven classes just 

 named, the Anamnionata as well as the Amnionata, agree 

 among one another in numerous characteristics, which dis- 



