THEIR EVOLUTION. 141 



of these forms the pit (T) in the temporal region of the 

 skull was larger than the socket of the eye, whereas in 

 recent crocodiles it is much smaller (Fig. 45), and in the 

 alligators may even disappear. A large number of these 

 Jurassic forms were of marine habits, and a few of them 

 attained enormous dimensions, the length of the skull of 

 one species falling not much short of five feet. A few 

 species are further peculiar in having altogether discarded 

 their bony armour en both surfaces of the body. 



Looking at crocodilians as a whole, it is perfectly evident 

 that they have advanced in complexity of organization 

 with the progress of time, the backwardly-placed internal 

 nostrils and ball-and-socket vertebrae of the modern types 

 being clearly an advance on the Jurassic forms. As, 

 however, is the case in many similar instances, the gradual 

 backward shifting of the internal nostrils presents a 

 problem difficult to understand, as it is hard to conceive 

 what advantage the species in which these nostrils were 

 situated in the middle of the palate had gained over 

 reptiles in which they were placed near the muzzle, the 

 completely backward position being apparently essential 

 in order that the mouth might be kept open under water. 



With regard to the general disappearance of the inferior 

 body-armour and the invariably increased development of 

 that on the back of the recent forms, it may be suggested 

 that, as most of the Jurassic crocodilians were of manne 

 habits, and probably swam far out to sea, it would have 

 been highly advantageous for them to have the lower sur- 

 face of the body protected from attacks from below, by 

 sharks and other creatures. On the other hand, since 

 modern crocodiles and alligators spend a considerable 

 portion of their time on the banks of rivers, and when in 

 the water are in the habit of reposing or crawling on the 

 bottom, it is obvious that the back is the portion which 

 requires especial protection. An explanation of the exist- 

 ence of an armour on the lower surface of the body in 

 the caimans is less easy to give, although it may be merely 

 an instance of the retention of an ancestral character. 



We may conclude this account by referring to some 

 interesting observations on the eggs and embryos of the 



