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.
Fig. 50. Preparation of part of the right mandibular ramus of Crocodilus palustris × ½. (Brit. Mus.)
| 1. tooth in use. | 3. symphysial surface of the |
| 2. fairly old germ of future | mandible. |
| tooth. |
Teeth have been detected in embryos of Trionyx, but otherwise no teeth are known to occur in Chelonia, or in Pteranodon (Pterosauria), while the anterior part of the jaw is edentulous in Iguanodon, Polyonax and some other Dinosaurs, and in Rhamphorhynchus.
ENDOSKELETON.
Vertebral column.