THE SKELETON OF REPTILES 43 



part there is only a single centrale, and even that is usually 

 lost in later reptiles. The third row, like the third row of the 

 carpus, had a distinct bone for each digit originally, but the fifth 

 one was very soon lost and has never reappeared. The structure 

 of the digits and number of bones are quite like those of the hands, 

 except that the fifth toe has four bones instead of three, that is, 

 the phalangeal formula was 2, 3, 4, 5, 4. As a rule in terrestrial, 

 reptiles, as in terrestrial mammals, the hind foot is more specialized 

 than the front ones. 



Most reptiles have an external covering or exoskeleton of horny 

 plates or scales or bony scutes. Homy scales are of course not 

 preservable as petrifactions, though in many instances their 

 actual carbonized remains or their impressions have been detected. 

 Such information comes only rarely, though doubtless in the 

 course of time we shall obtain it for most extinct reptiles. In the 

 mosasaurs, for instance, very perfect impressions showing the 

 detailed structure of the scales have been frequently found. Similar 

 impressions were long since observed by Lortet in Pleurosaurus, 

 and in not a few dinosaurs impressions of most wonderful perfec- 

 tion have been found. It is only in the water reptiles, probably, 

 that all external coverings tended to disappear. 



Bony dermal plates or scutes are less common among reptiles, 

 though by no means rare. The turtles, as is well known, are 

 almost completely inclosed in such an exoskeleton, bones which 

 have coalesced more or less to form a box or carapace within 

 which the head and limbs may be withdrawn for protection. 

 In the modern crocodiUans also the body is more or less protected 

 by small bone plates forming rows on the back and sometimes 

 on the under side. The ancient phytosaurs had similar plates. 

 Not a few of the dinosaurs were more or less covered with bony 

 scutes and sometimes with large bony plates or spines. Some 

 modem lizards have bony plates over the body instead of horny 

 scales. 



