GENERAL CHARACTERISTICS. 



separated from Birds by the absence of feathers; the appendages of the outer 

 layer of the skin being in the form either of overlapping horny scales, or of large 

 shields uniting by their opposed edges. Moreover, all known Reptiles differ from 

 Birds in having more than three digits in the fore-limb ; while in no cases are 

 the collar-hones fused into a furcula, as they are in all flying Birds. A further 

 distinction is to be found in connection with the circulatory system, the blood of 

 all existing Reptiles being cold, while the aorta, or great propelling blood-vessel of 

 the heart is double, and 

 crosses both branches (in- 

 stead of only the left branch) 

 of the windpipe. It will 

 be obvious, however, that 

 these two last characters 

 cannot be verified in the 

 case of extinct Reptiles, 

 among which it is quite 

 probable that there may 

 have been some in which 

 the blood was warm. A 

 similar remark will apply 

 to the absence among living 

 Reptiles of those ramifica- 

 tions of the bronchial tubes 

 throughout the body, which 

 form such a characteristic 

 feature in the structure of 

 Birds. As additional features 

 in the skeleton, it may be 

 noticed that Reptiles never 

 have the terminal faces of 

 the vertebrae saddle-shaped ; 



while in those forms in which the number of toes in the hind-limb is reduced to three, 

 the metatarsal bones do not unite to form a cannon-bone in conjunction with the 

 lower row of bones belonging to the ankle-joint. Then, again, with the exception 

 of one remarkable extinct group, Reptiles, as a rule, are characterised by the three 

 bones of the pelvis remaining distinct from one another through life ; whereas in all 

 existing birds they are welded together. There are likewise differences in regard 

 to the form and structure of the breast-bone and sacrum, into the consideration of 

 which it will be unnecessary to enter in this work. 



Diversity of I n marked contrast to the uniformity in appearance and structure 



Form and characterising Birds, the various groups of Reptiles differ widely from 

 cture> one another, both as regards external form and internal structure. 

 Externally, a lizard, a snake, and a tortoise present the most marked differences in 

 general appearance among living members of the order ; while among extinct types 

 there were some which walked on their hind-limbs alone, after the manner of Birds, 

 and others having their fore-limbs modified into wings and the digits connected 



THE BONES OP THE LEFT SIDE OF THE PELVIS OF AN EXTINCT 

 DINOSAURIAN REPTILE (^ Iiat. size). 



il, ilium : p, pubis ; is, ischium. — After Marsh. 



