THE SKELETON IN REPTILES. THE TEETH. 273 



The method by which they are attached to the bones varies 

 much. Sometimes as in Iguana and some other lizards, they 

 are pleurodont 1 , sometimes they are acrodont 1 , as in the Rhyn- 

 chocephalia, Pythonomorpha, Ophidia and some Lacertilia such 

 as Agama. Again they may be set in a continuous groove 

 as in the Ichthyosauria and young Crocodilia. Finally the 

 teeth may be thecodont or placed in distinct sockets as in the 

 Theromorpha, Sauropterygia, adult Crocodilia, Sauropoda and 

 Theropoda. In Iguanodon the teeth are set in shallow sockets 

 in a groove one side of which is higher than the other ; the 

 method of attachment thus shows points of resemblance to 

 the thecodont condition, the pleurodont condition, and that 

 met with in the Ichthyosauria. 



In Ichthyosaurus the teeth are marked by a number of 

 vertical furrows, and it is from a furrow of this nature greatly 

 enlarged and converted into a tube that the channel down 

 which flows the poison of venomous snakes is derived. 



In most reptiles the dentition is more or less homodont. 

 The only reptiles in which a definite heterodont dentition is 

 known are the extinct Theromorpha, and in them the teeth 

 vary greatly. Thus Udenodon is toothless, the jaws having 

 been probably cased in a horny beak. In Dicynodon the jaws 

 are likewise toothless with the exception of a pair of perma- 

 nently growing tusks borne by the maxillae. Dicynodon is the 

 only known reptile whose teeth have permanently growing 

 pulps. In Pariasaurus the teeth are uniform and very nume- 

 rous, and though placed in distinct sockets are ankylosed to 

 the jaw. In Galesaurus and Cynognathus three kinds of teeth 

 can be distinguished, slender conical incisor-like teeth, large 

 canine -like teeth, and cheek teeth with two or three cusps. 

 The teeth in Galesaurus are confined to the jaws, in Placodus 

 and its allies, however, large flat crushing teeth are attached to 

 the palatines as well as to the jaw-bones, and in Pariasaurus 

 the vomer, palatine and pterygoid all bear teeth as well as the 



1 These terms are defined on p. 199. 

 B. 18 



