140 REPTILES 



numerous types of dentition occurring in reptiles will be noticed 

 but briefly ; attention being directed only to some of the more 

 remarkable types. 



Before mentioning some of the leading types of dentition, 

 reference may be made to one curious adaptive modification 

 connected with food which occurs in the small South African 

 snake known as Dasypeltis scabra, which, as stated on page 

 50, feeds largely upon birds' eggs. On the page referred to 

 it is likewise stated that this snake crushes the eggs upon which 

 it subsists by means of knob-like projections from the spines on 

 the lower sides of the vertebrae of the neck which project into 

 the gullet. Now the button-like extremities of these vertebral 

 spines are furnished with caps of enamel, so that both in struc- 

 ture and function they are comparable to teeth. 



Passing on to the consideration of teeth, it may be mentioned 

 that there are three distinct modes in which reptilian teeth are 

 attached to the jaws. Firstly, as in the ichthyosaurs, they may 

 be implanted in a continuous open growth in the jaws ; and 

 from this there is but a step, by the development of partitions in 

 the groove, to their implantation in distinct sockets (the codont 

 type), as in crocodiles and the extinct megalosaur. Secondly, 

 there occurs in certain lizards, as well as in the tuatera, what is 

 known as the acrodont type of attachment, that is to say, when 

 the teeth are welded to the summit of the jaws. Finally, we 

 have the pleurodont type, as exemplified in the iguanas, in 

 which the teeth are fused by one side of their bases to the inner 

 side of a parapet formed by the elevation of the outer wall of 

 the jaws. The lizards which exhibit the acrodont type include 

 the stellions (Agamidce) and some of the amphisbaenas ; while 

 this type reappears in the chamaeleons, and as already said in 

 the tuatera. 



The socket, or thecodont, type of dentition appears gener- 

 ally associated with carnivorous habits, as witness crocodiles, 

 megalosaurs, pterodactyles, and plesiosaurs ; the teeth in all 

 these cases being of a more or less conical (crocodiles) or com- 

 pressed (megalosaurs) type, although they may become trihedral, 

 as in those specialised plesiosaurs known as pliosaurs. The 

 thecodont type also occurs in the carnivorous, or theriodont, 

 mammal-like reptiles, in which they become differentiated into 

 incisors, canines or tusks, and cheek-teeth, as in mammals. On 



