Teeth may be present not only on the jaw-bones, but as in many _Squamata_, also on the palatines, pterygoids or vomers. 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[92], as in the Rhynchocephalia, 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 permanently 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 numerous, 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 jaw bones. The upper jaw of _Sphenodon_ and other Rhynchocephalia is provided with two parallel rows of teeth, one borne on the maxillae and one on the palatines, the mandibular teeth bite in a groove between these two rows. The bone of the jaws in _Sphenodon_ is so hard that when the teeth get worn away, it can act as a substitute. In the young _Sphenodon_ the vomers bear teeth, as they do also in _Proterosaurus_. There is generally a continuous succession of teeth throughout life, the new tooth coming up below, or partly at the side of the one in use, and causing the absorption of part of its wall or base. In this way the new tooth comes to lie in the pulp cavity of the old one. This method of succession is well seen in the Crocodilia.