224 THE ANATOMY OF VEETEBEATED ANIMALS. 



The Lacertilia all possess teeth, which may be confined 

 to the premaxillse, maxillae, and dentary piece of the man- 

 dibles ; or may, in addition, be developed on the palatine and 

 pterygoid bones. These teeth are simple in sti'uctnre, and 

 their crowns have very various forms, being sometimes 

 shai-p and conical {Monitor) ; or bladelike, with serrated 

 edges {Iguana'); or with broad, crushing, and spheroidal 

 crowns {Cyclodus). As a general rule, the teeth become 

 ankylosed to the adjacent bone with age ; and in the 

 upper and lower jaws they thus become attached, either by 

 their sides to the parapet of the jaw, when the dentition is 

 said to be pleurodont ; or by their bases to the summit of 

 the parapet, when the dentition is acrodont. The extinct 

 Protorosauria are said to be thecodont, or to have the teeth 

 lodged in alveoli. New teeth are usually developed at the 

 bases of the old ones. 



The Lacertilia are divisible into numerous groups, the leading dis- 

 tinctive characters of which are exhibited in the following table: — 

 I. — The pterygoid and quadrate bones united. 

 A. A columella and an interorbital septum in the skull. 

 Kioriocrania (Stannius). 



a. Amphicoelous vertebras (A'. Amphiccelia.) 



a. Dentition acrodont or pleurodont. 



1. Ascalabota. 



2. Rhynchocephala. 



3. Homaosauria.* 



h. Dentition thecodont (?). 



4. Protorosauria.* 



b. Procoelous vertebr<E {K. jtroccelia.) 



a. Kot more than nine cervical vertebrae. 



a. The nasal bone, single. 



5. Platynota. 



b. The nasal bones, two. 



1. The integument of the head not 



covered with epidermic plates. 



6. Eunota. 



2. The integument of the head 



covered with epidermic plates. 



7. Lacertina. 



8. Chalcidea. 



9. Scincoidea. 



b. More than nine cervical vertebrae. 



10. Dolichasauria* 



11. Mosasauria. 



* The columella has not been observed in these groups. 



