XIII 



PHYLUM CHORDATA 



561 



the teeth that are placed opposite to these. The upper canine (s.) is 

 the most anterior tooth of the maxilla situated on or immediately 

 behind the premaxillo-maxillary suture, and has usually a charac- 

 teristic shape. The lower canine is the tooth which bites in front 

 of the upper canine. The pre-molars (p.) are distinguished from 

 the molars by having milk predecessors (d.m.), but the first pre- 

 molar is, except in the Marsupials, nearly always a persistent 

 milk-tooth ; the molars (m.) have no teeth preceding them, and 

 are sometimes looked upon as persistent teeth of the first set. 



FIG. 1185. Upper and lower teeth of one side of the mouth of a Dolphin (Lagenorhynchus), 

 illustrating the homodont type of dentition in a Mammal. (After Flower and Lydekker.) 



The various sets of teeth are also usually distinguishable by 

 their shape. As a rule the incisors have cutting edges ; the 

 canines are pointed and conical ; the pre-molars and molars have 

 broad surfaces with ridges and tubercles for crushing the food, 

 and may have from two to four roots. 



The simplest form of molar tooth (which occurs, however, only in 

 certain extinct forms) is that of a simple cone, or a cone with two 

 small accessory processes or cusps. Almost as primitive is the 

 type of tooth termed triconodont (likewise occurring only in a 

 few extinct Mammals), in which there are three equal conical 

 cusps set in a straight line, the upper teeth biting on the outer 

 side of the lower From the triconodont is derivable the trituber- 

 culate molar, in which the free surface of the tooth presents three 

 cusps or tubercles arranged in a triangle, the apex of which is 

 internal in the upper, external in the lower jaw. In the upper 

 molar the inner cusp is termed the protocone, the antero-external 

 the paracone, and the postero-external the metacone. These terms 

 are modified in the case of the molars of the lower jaw, the 

 equivalent of the protocone, here external, being termed the 

 protoconid and the others paraconid and metaconid respectively. 

 This trituberculate type of molar is usually complicated by various 

 additions and modifications accessory cusps being added, together 

 with ridges or folds connecting the cusps together. The resulting 

 complex tooth may be modified to act as a cutting (secodoni) or a 

 crushing (bunodont) molar. A modification of the bunodont molar 

 is brought about by the cusps, instead of retaining their conical 

 form, being drawn out into the shape of crescents (selenodont). 



