GENERA L CHA RA C TERIS TICS 



2363 



such as certain tortoises, have the same number of joints in each toe as Mammals, 

 that is to say, two in the first toe, and three in each of the others, in the 

 greater majority there is a departure from this simple arrangement. In the lizards, 

 for instance, the number of joints in the toes (reckoning from the first to the fifth 

 digit) is 2, 3, 4, 5, 3 in the fore-limb, and 2, 3, 4, 5, 4 in the hind-limb; while in 

 crocodiles, where there are but four toes in the latter, the numbers are respectively 

 2, 3, 4, 4, 3, and 2, 3, 4, 4. In this increasing number of joints in the toes from 

 the first to the fourth, such reptiles approximate to birds. 



As regards their soft internal parts, Reptiles are characterized by the low 

 development of their brains, which, in conjunction with their cold blood, accounts 

 for the generally sluggish movements of their existing representatives. With the 

 exception of the 

 crocodiles, Reptiles 

 differ from Birds 

 in that the heart 

 has only three, in 

 place of four, com- 

 plete chambers, 

 thus causing the 

 freshly oxygen- 

 ated blood return- 

 ing from the lungs 

 to be mixed with 

 the effete blood 

 which has trav- 

 ersed the body. Even in crocodiles, where the heart has practically four cham- 

 bers, the fresh and effete blood is partially mingled, owing to a communication 

 between the vessels just outside the heart. Like Birds, Reptiles never have a midriff 

 completely separating the cavity of the chest from that of the abdomen. 



Reptiles having come into existence at an earlier period than either 



Mammals or Birds, and attaining an enormous development during 

 tribution epochs when both those groups were but feebly represented, it would 



be only natural to expect that they should have suffered to a much 

 greater extent by the extinction of types with the lapse of time. As a matter of 

 fact this is found to be the case; the number of existing orders of Reptiles being 

 now but four (of which one is represented by only one or two species), whereas, if 

 we include the extinct types, at least nine orders may be recognized. These nine 

 orders, of which the extinct ones are indicated by asterisks (*) may be named and 

 arranged as follows, viz. : 



i. CROCODILES Crocodilia. 

 *2. DINOSAURS Dinosauria. 

 *3. FLYING DRAGONS Ornithosauria. 



4. TORTOISES and TURTLES Chelonia. 

 *5. PLESIOSAURIANS Plesiosauria. 



FRONT AND BACK VIEWS OF A VERTEBRA OF A SNAKE. 

 zi indicates the additional articular process which is received into the cavity zi l . 



