378 REPTILIA— ORDER I.—CROCODILIA. 



the windpipe, in reptiles it is double, and passes over both branches of the 

 latter. Then, again, no reptiles arc known in which the number of digits 

 in the fore-limb is reduced to three ; while in no case are the two collar- 

 bones fused together to form the forcula, or merrythought, which consti- 

 tutes such a striking feature in the skeleton of a bird. Other minor differences 

 might be pointed out, but the foregoing are amply sufl&cient to distinguish 

 a bird from a reptile. In common with the egg-laying mammals, both groups 

 produce their young from eggs ; but in certain reptiles the young are retained 

 so long within the body of the parent that they are bom alive into the world. 



From the lower vertebrates, that ia to say frogs, salamanders, newts, and 

 fishes, reptiles differ in that they are never provided with gills at any period 

 of life, and when first hatched are similar in general form and structure to 

 the adult, or, in other words, do not undergo a metamorphosis. 



Whereas birds are remarkably alike in general appearance and internal 

 structure, reptiles present great diversity in these resijects, and the distinc- 

 tion between the various orders of the latter are much more strongly marked 

 than is the case with those of the former. If extinct types be taken into 

 account, these diversities are still more strongly accentuated, and there are 

 certain fossil forms which walked on their hind legs alone, while others were 

 capable of flying in the same manner as a bird. For this reason the presence 

 or absence of the power of flight is not given as one of the distinctions 

 between birds and reptiles. 



Geologically, reptiles are much older than either mammals or birds, and 

 consequently a largo number of their orders are extinct, whUe one is repre- 

 sented only by a solitary survivor. The existing orders are as follows : — 

 I. Crocodilia (Crocodiles, Alligators, and Garials). 

 11. Chelonia (Tortoises and Turtles). 



III. Squamata (Lizards, Chameeleons, and Snakes). 



IV. Ehynchocephalia (New Zealand Tuatera.) 



In consequence of their greater antiquity, and the numerous migrations 

 which they have undergone during early periods of the earth's history, 

 reptiles do not agree with mammals in respect to their geographical distribu- 

 tion ; so that the realms and regions into which the earth is divided from the 

 evidence of the present and past distribution of mammals do not hold good 

 for reptiles. Some of the leading features in the distribution of the orders 

 and sub-orders of tlie present class are mentioned in the sequel. 



ORDER I.—CROCODILIA. 



Crocodiles, Alligators, and Garials. 



Nobody can mistake a crocodile or alligator, or even their long-snouted 

 allies, tlio garials, for any other reptile ; and consequently, although they are 

 nearly allied to certain extinct forms, there is no necessity here for givmg an 

 elaborate and detailed definition of the order. Great lumbering brutes, 

 lying like logs in the water, but waking into sudden and unexpected activity 

 directly living prey comes within their reach, crocodiles and alligators are 

 sjiccially characterised by the rows of large shield-like horny plates covering 

 the back and parts of the neck, and in some cases also the under surface oi 



