GENERAL CHARACTERS 13 



of that segment, does not seem to have been ascertained. But 

 here, again, it is clear that there must either have been some 

 theromorphs with a mammal-like tarsus, or mammals with a 

 tarsus of the reptilian type. 



As regards other features of the skeleton, it may be noted 

 that a true sternum, or breast-bone, exists, and that in many 

 instances the ribs are furnished with uncinate processes, that is 

 to say from the posterior side of each rib proceeds an oblique 

 process which overlaps the succeeding rib. These same uncinate 

 processes occur in birds, and may be inherited from reptiles. 



The few features of the soft parts of reptiles to which space 

 permits of allusion may be very briefly dismissed, since it is 

 generally impossible to say whether they were common to the 

 extinct types. Firstly, it is important to mention that epidermic 

 scales — or their equivalents, large horny plates — form the char- 

 acteristic covering of reptiles, and that the feathers of birds and 

 the hairs of mammals are alike conspicuous by their absence. 

 Here, in connection with the evolution of birds from reptiles, a 

 great difficulty presents itself, for it seems almost impossible to 

 imagine a scaly bird, while if we ate driven to postulate a 

 feathered reptile we are confronted with the difficulty of as- 

 signing an adequate reason for the development of such a type 

 of covering. Less difficulty is connected with the idea of a 

 scaly mammal, or even of a hairy reptile. 



Cold blood is characteristic of all living reptiles, as is also 

 the fact that the corpuscles of this fluid (like those of birds, 

 but not of mammals) are furnished with a nucleus. The heart 

 is in the main tripartite, but of such a type that the four- 

 chambered organ of both birds and mammals might be easily 

 evolved therefrom. Similarly, the presence of a right and a left 

 aortic arch, permits us to see how, by the suppression of one or 

 the other, the mammalian and the avian type of circulatory 

 system have been respectively evolved. The gills of the young 

 of the ancestral amphibians have been completely lost in reptiles 

 so that, as in the two higher vertebrate classes, respiration, 

 takes place by means of lungs alone. The excretory organs, 

 as in birds and the lower mammals, discharge into a common 

 outlet — the cloaca. During development the embryo is en- 

 veloped in the two membranes respectively known as the 

 amnion and the allantois; and the egg is of what is termed the 



