TEETH. 43 



tooth is curved backwards, forming a canal along posterior 

 surface of tooth, which communicates with duct from poison 

 gland. In Lacertilia, Iguanodon partially, and in Alligator 

 entirely inserted within alveolar groove. Peculiarity of 

 Dicynodan, in which there is but a single large tooth on 

 either side of superior maxillary bone. 



Mammalia. Teeth confined to intermaxillary, superior, 

 and inferior maxillary bones ; each is enclosed within a bony 

 socket, and placed in a single series immediately behind the 

 lips; and are the only teeth in Yertebrata which may possess 

 more than a single fang. They are of a definite number, and 

 appear in sets (when they are divided into two series, the 

 deciduous or milk teeth, and the permanent teeth), as in the 

 majority of mammals, or are of one set only, as in Cetacea. 

 The deciduous teeth are always fewer in number than the 

 permanent; and in Marsupialia are confined to a single tooth 

 (posterior molar) on either side in each jaw. (Flower.) The 

 deciduous teeth are sometimes lost prior to maturity (in 

 utero in some Rodentia), or oftener at a comparatively early 

 period after birth. 



Teeth are divided into cutting teeth (incisors), tearing 

 teeth (canines), and grinding teeth (molars), and always 

 hold fixed relations to the character of food. Thus the pre- 

 ponderance of a conical form indicates a carnivorous habit. 

 With it, the canines are greatly developed, the incisors, 

 pointed; the molars are acutely cusped, and present sur- 

 faces adapted, in opposition, to divide the food by a scissor- 

 like motion. In extreme carnivorous forms, the molars are 

 rudimentary, as in Felis (cat), or are absent, as in Macliairodus 

 (sabre-toothed tiger). The preponderance of a grinding form 

 indicates an herbivorous habit. With it, the molars, always 

 present, are large, flat, and yield either a tuberculatcd sur- 

 face, as in Ursus (bear), or a flattened, uneven one, marked 

 by ridges of enamel, as in Equus (horse). Every molar is con- 

 structed upon a common plan. The dentinal centre is capped 

 superiorly by enamel, and the whole structure then sur- 

 rounded by a cemeutal membrane. Both enamel and ce- 

 mental membrane follow the curvings of the surface of the 



