GENERAL CHARACTERISTICS 



2361 



day, and in some they are believed to have occupied the extraordinary position 

 shown in the figure on the preceding page. 



Still more remarkable differences exist with regard to the form and structure of 

 the teeth; which, instead of being, as in the two preceding classes, strictly confined 

 to the borders of the jaws, may be spread over the entire palate. In spite, how- 

 ever, of this diversity of form, the teeth of Reptiles differ from many of those of the 

 majority of Mammals in that they are never implanted in the jaws by two or more 

 roots; while in no cases are their crowns complicated by the presence of infoldings 

 of enamel. The simplest type of reptilian tooth is in the form of a cone; such 

 conical teeth being confined to the margins of the jaws, where, as among crocodiles, 

 they may be implanted in distinct sockets, or, as in the extinct fish lizards, in an 

 open groove. In other 

 cases, as among lizards, 

 teeth of the same gen- 

 eral type may be united 

 by a bony deposit either 

 to the summit or to one 

 side of the margin of 

 the jaw. In place of 

 the one regular replace- 

 ment, characterizing the 

 anterior teeth of the 

 majority of Mammals, 

 the teeth of most Rep- 

 tiles are replaced irregu- 

 larly and continuously 

 throughout life; the 

 successional teeth grow- 

 ing up beneath the bases 

 of those in use, and gradually causing an absorption of their roots. When teeth 

 are distributed over the whole or a greater portion of the palate, they generally as- 

 sume a more or less flattened and bean -like shape, so as to form a kind of pave- 

 ment in the mouth, as shown in the accompanying figure of the under surface of 

 the skull of an extinct reptile. Between conical and pavement-like teeth there are 

 various intermediate grades, some of which will be referred to in the sequel. It is, 

 however, by no means all members of the class that are provided with teeth; the 

 tortoises and turtles being living examples of the total loss of these organs, and the 

 consequent conversion of the jaws into horn-clad beaks. Certain representatives of 

 the extinct flying dragons were likewise devoid of teeth, and as in these forms the 

 horn-covered jaws were long and narrow, the resemblance to the beak of a bird be- 

 comes most marked. 



It has already been stated that the vertebrae of Reptiles never articulate by 

 means of those saddle-shaped surfaces so characteristic of Birds. They present, 

 however, great diversity of structure in this respect. In some cases, for instance, 

 as in the fish lizards, the bodies or central portions of the vertebrae are very short 



CONICAL TOOTH OF AN 

 EXTINCT PLESIOSAU- 

 RIAN REPTILE. 



UNDER SURFACE OF SKULL OF AN 

 EXTINCT REPTILE (Cyamodus) WITH 



PAVEMENT-LIKE TEETH ON THE 

 PALATE. 



