204 CHARACTERS OF VERTEBRATE ANIMALS 



It may be well at this point to summarize the chief features, all 

 of which have been illustrated in the foregoing description of a 

 Lizard, in which these five groups agree, or, in other words, to 

 give a definition of the Reptilia. 



Reptiles are cold-blooded Vertebrates defended by an exo- 

 skeleton composed of either epidermic scales, or bony dermal 

 scutes, or it may be of both. In those forms which possess well- 

 developed extremities, there are always more than three digits 

 in the fore-limb, and commonly five in the hind. The skull is 

 jointed on to the backbone by a single occipital condyle, each half 

 of the lower jaw consists of several bones, and the surfaces of 

 the vertebral centra are not saddle - shaped. The sacrum is 

 made up of two vertebrse. The interclavicle, when present, 

 does not fuse with surrounding parts, and in existing forms the 

 ilia do not extend far in front of the acetabula, both pubes and 

 ischia unite together ventrally, and the tarsus fuses neither with 

 the tibia nor with the metatarsus. The ankle-joint is in the 

 ■middle of the tarsus. Conical teeth are usually present (except 

 in Chelonia) and the gut ends in a cloaca. The heart possesses 

 two auricles but (except in Crocodilia) only one ventricle, and 

 in all cases the body is partly supplied with mixed blood. There 

 is never less than one pair of aortic arches. The brain is small, 

 and the optic lobes are placed close together on its upper side. 

 Breathing is in no case effected by means of gills. 



There are many points of agreement between Reptiles and 

 Birds, and this is especially striking if extinct forms are taken 

 into consideration. Those characters which are printed in italics 

 in the above definition are common to both groups, which are 

 often associated together into a larger division, the Sauropsida, 

 contrasting with Mammals on the one hand and Ichthyopsida 

 (Amphibia plus Fishes) on the other. 



Order i. — Crocodiles (Crocodilia) 



Crocodiles and Alligators (fig. 133) much resemble gigantic 

 lizards in appearance, and agree with these in many particulars 

 of structure, though decidedly of higher type. As is well known, 

 these animals spend a large part of their time in water, and 

 some of their peculiarities of structure are related to the aquatic 

 habit. The large and hideous head provided with formidable 



