66 ZOOLOGY SECT. 



The range of variation in the whole of the six classes included 

 in the division is, in fact, considerably less than in many single 

 classes of Invertebrata for instance, Hydrozoa or Crustacea. 

 Hence, while the plan hitherto adopted of treating the group class 

 by class will be followed, it will be found convenient to begin by 

 devoting a considerable space to a preliminary account of the 

 Craniata as a whole, since in this way much needless repetition 

 will be avoided. 



The Craniata include the following classes and sub-classes : 



CLASS I. CrcLosTOMi, 

 Including the Lampreys and Hags. 



CLASS II. PISCES, 

 True Fishes, which are again divisible into 



Sub-class 1 . Elasm obranchii, 

 Including the Sharks and Rays. 



Sub-class 2. Holocepkali, 



Including the Cat-fish (Chimcera) and the Elephant-fish 



(Callorhynchus). 



Sub-class 3. Teleostomi, 



Including the bony Fishes, such as Perch, Cod, Trout, &c., and the 

 Sturgeons and their allies. 



Sub-class 4. Dipnoi^ 

 Mud-Fishes. 



CLASS III. AMPHIBIA, 



Including Frogs, Toads, Newts, and Salamanders. 



CLASS IV. REPTILIA, 

 Including Lizards, Snakes, Crocodiles, Turtles, and Tortoises. 



CLASS V. AVES, 

 Birds. 



1 The animals included in Classes I. and II. are all " Fishes " in the broad 

 sense of the word. 



