the Fossil Jaws of Stonesjield. 651 



In the jaws of animals provided with a dental appara- 

 tus, the teeth have either both crowns and roots simple, or one 

 of the two complex ; or else the complex structure in both is 

 associated ; the last is most generally the case in the mo- 

 lar teeth, especially the posterior ones ; while a simple struc- 

 ture of both these parts is the usual condition of the inci- 

 sors and canines. 



In this class there has never yet been found a dental sys* 

 tern in which all the teeth are complex in both parts. A 

 small number of species have them all simple, both as regards 

 the crown and the root, as in all the Cetacea, and some seals ; 

 but in the greater number both kinds are associated, that is 

 teeth of simple structure placed in front and complex behind. 



In animals of this class all the teeth have the root, whether 

 simple or complex, buried to a greater or less depth in an en- 

 tire socket, and these sockets communicate with the dental 

 canal, through which pass the nervous filaments that com- 

 municate with the teeth. 



With the exception of the whales and the Ornithorhynchi, 

 all the Mammalia resemble each other as regards the system 

 of dentition. 



In the Reptilia there is much more variation in this part 

 of their organization. In fact, if the teeth, which are almost 

 always simple, may yet sometimes have sub-complex crowns*, 

 as for example, among the iguanas, they are constantly simple 

 in their roots ; at least I have no example of a contrary nature, 

 if indeed there be any true root besides their base, in the 

 greatest number of cases. 



These teeth differ so sometimes that we are able to dis- 

 tinguish them as incisors, canines, and molars, though these 

 last are almost always absolutely simple. 



But the greatest variation presented by the dental system 

 of Reptiles is observed in its connexion with the jaws, in 

 Which relation we may indeed distinguish four distinct kinds. 



In the first there is a hollow alveolus entirely surrounded by 

 the bone, and in which the tooth is loosely implanted, as is 

 seen for example in the crocodile. 



In the second, the alveoli holding the teeth are also com- 

 prised between the two plates of the jaw, but in a manner so 

 closely united, that the tooth seems to form a portion of the 

 jaw itself, denticulating, as it were the edge, as is seen in the 

 geckoes, chameleons, dragons (Dracones), and even the 

 agamas. 



The third mode, is that in which there is no alveolus, but 

 where the teeth are disposed between the internal wall of the 

 jaw and the " membrane gyngivale ;" this is the case in the 



Vol. II.— No. 24, n. s. 3 t 



