GENERAL CHARACTERISTICS. 5 



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 

 general 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 re- 

 placement, characteris- 

 ing the anterior teeth of 

 the majority of Mam- 

 mals, the teeth of most 

 Reptiles are replaced 

 irregularly and continu- 

 ously throughout life ; 

 the successional teeth 



growing up beneath the bases of those in use, and gradually causing an absorp- 

 tion of their roots. When teeth are distributed over the whole or a greater 

 portion of the palate, they generally assume a more or less flattened and bean- 

 like shape, so as to form a kind of pavement in the mouth, as shown in the 

 accompanying figure of the under surface of the skull of an extinct reptile. 



CONICAL TOOTH OP AN 

 EXTINCT PLESIO- 

 SAURIAN BEPTILE. 



UNDER SURFACE OP SKULL OP AN EXTINCT 



reptile {Cyamodus), with pavement - like 



TEETH ON THE PALATE. 



LEFT SIDE OF THE SKULL OF A BEAKED FLYING DRAGON (J nat. size). 



a, vacuity in front of the eye ; b, socket of the eye ; c, occipital spine ; d, angle of lower jaw ; e, extremity of 

 upper, and e', of lower jaw ; q, articulation of the skull proper with the lower jaw ; s, point where the two branches 

 of the lower jaw diverge. — After Marsh. 



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 conver- 



