544 



ZOOLOGY 



SECT. 



the upper jaw are to be distinguished as being the teeth that are 

 lodged in the premaxillse ; the incisors of the lower jaw are 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 pre-maxillo-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- 





FIG. 1197. 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.) 



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. 

 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 ac- 

 cessory processes or cusps. 

 Almost as primitive is the 

 type of tooth termed tri- 

 conodont (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 trituberculate 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 equi- 



FIG. 1198. Teeth of Bandicoot (Perameles). 

 (After Owen.) 



