10 REPTILES AND BIRDS. 



upon their mode of respiration, the volume of their eggs, and their 

 colour. 



Numerous systems have since appeared in France, Germany, 

 and England ; but we shall best consult the interest of our readers 

 by briefly describing the classification adopted by Owen in his 

 classical work on the vertebrata, although no two schemes agree in 

 all their details. 



The two great classes Batrachians and Reptiles, include -a 

 number of animals which are neither clothed with hair, like the 

 Mammalia, covered with feathers like the Birds, nor furnished 

 with swimming fins like Fishes. The essential character of Reptiles 

 is, that they are either entirely or partially covered with scales. 

 Some of them for instance, Serpents move along the ground 

 with a gliding motion, produced by the simple contact and ad- 

 hesion of the ventral scales with the ground. Others, such as 

 Tortoises, Crocodiles, and Lizards, move by means of limbs ; but 

 these, again, are so short, that the animals, with very few exceptions, 

 appear only capable of crawling slowly. Again, some of this class 

 are only furnished with feet in the pectoral region; but this is the 

 exception. The locomotive organs in Serpents are the vertebral 

 column, with its muscles, and the stiff epidermal scutes crossing the 

 under surface of the body. " A Serpent may, however, be seen to 

 progress," says Sir Rich. Owen, " without any inflection, gliding 

 slowly and with a ghost-like movement in a straight line ; and if the 

 observer have the nerve to lay his hand flat in the reptile's course, 

 he will feel, as the body glides over the palm, the surface pressed as 

 it were by the edges of a close-set series of paper knives successively 

 falling flat after each application." In some, as in various Lizards, 

 the limbs acquire considerable strength. 



There is one genus of small Lizards, known as the Dragons (Draco\ 

 whose means of progression present an exception to the general rule. 

 Besides their four feet, these animals are furnished with a delicate 

 membranous parachute, formed by a prolongation of the skin on the 

 flanks, and sustained by the long slender ribs, which permits of their 

 gliding through the air upon their prey from a considerable height. 



Batrachians, again, differ from most other Reptilia by being 

 naked; moreover, most of them undergo certain metamorphoses. 

 In the first stage of their existence they lead a purely aquatic life, 

 and breathe by means of gills, after the manner of fishes. Young 

 Frogs, Toads, and Salamanders, which are then called tadpoles, have 

 at that stage no resemblance whatever to their parents in structure. 

 They are little creatures with slender, elongated bodies, destitute of 



