II.] THE COMMON FROG. 19 



The Vertebrata are divided into five great Classes : 

 — (I.), Mammalia (Man and Beasts); (II.), Aves 

 (Birds); (HI.), Reptilia (Reptiles, i.e. Crocodiles, 

 Lizards, Serpents, and Tortoises) ; (IV.), Batrachia 

 (Amphibians, i.e. Frogs, Toads, Efts, &c.) ; and (V.) 

 Pisces (Fishes), 



Of these five classes Birds and Reptiles are classed 

 together in a larger group called Sauropsida^ because 

 they present so many structural resemblances. Simi- 

 larly Amphibians and Fishes are grouped together, 

 and to their united mass the common term Ichthy- 

 opsida is applied. 



The orders into which the two classes, Mammalia 

 and Aves (beasts and birds), are divided, may here 

 be neglected, as we shall have little to say respecting 

 them in the following pages. There are, however, 

 about twelve orders of beasts, and probably some 

 fourteen of birds. 



The class of Fishes has been subdivided into five 

 Orders. 



1. Elasmobranchii (the sharks and rays, or highly 

 organised cartilaginous fishes). 



2. Ganoidei, an important order, containing many 

 extinct forms, and a few very varied existing ones, 

 such as the mud-fish (Lepidosiren), Ceratodus, and 

 the sturgeon. 



3. Teleostei, the ordinary or bony fishes, such 

 as the carp, sole, perch, &c., and containing a re- 

 markable group called Siluroids, as also the curious 

 little sea-horse — Hippocampus. 



4. Marsipobranchii (the lamprey and myxine, or 

 lowly organised cartilaginous fishes). 



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