LARGER GROUPS IN THE VERTEBRATA. 197 
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- 
