GENERAL CHARACTERISTICS 



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a 



rows of small bones entering into the composition of that part of the skeleton. In 

 producing their young from eggs (sometimes retained within the body of the parent 

 until hatched), Reptiles resemble not only Birds, but likewise the lowest Mammals; 

 with which they also agree in the nature of the investments surrounding the 

 embryo. As regards the distinction between the two groups, Reptiles are broadly 

 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 bones 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 charac- 

 teristics cannot be verified 

 in the case of extinct Rep- 

 tiles, 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 fea- 

 tures 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 canon bone in conjunction with the lower row of 

 bones belonging to the ankle joint. Then, again, with the exception of one re- 

 markable extinct group, Reptiles, as a rule, are characterized 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 breastbone and sacrum, into the consideration of which it 

 will be unnecessary to enter in this work. 



THE BONES OF THE LEFT SIDE OF THE PELVIS OF AN 

 EXTINCT DINOSAURIAN REPTILE. 



(One-twelfth natural size.) 

 it. ilium; p. pubis; is. ischium. After Marsh. 



