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. — Cyclostomi, 

 Including the Lampreys and Hags. 



Class II. — Pisces, 

 True Fishes, which are again divisible into 



Sul-class 1 . — Elasmohranchii, 

 Including the Sharks and Rays. 



Sub-class 2. — Holoccphali, 



Including the Cat-fish (Chima:ra) and the Elephant-fish 



( Ccdlorhynchus'). 



Sub-class 3. — Tcleostomi, 



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



Sturgeons and their allies. 



Suh-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. an<l II. are all "Fishes" in the broad 

 sense of tlie word. 



