DENTITION. 503 



In the same way the molars are supposed to be cut off at the hind end of the 

 series also, but if the premolars are incomplete it is the anterior which are 

 supposed to be missing. No doubt these are rules in some cases, but there 

 are many cases in which they do not apply, as will be seen in the account 

 of the different orders. 



The anterior milk molars as a rule resemble premolars in the 

 permanent dentition, while the hind milk molars are molar-like 

 in form. 



The dentitions of the Cetacea, Edentata, and Marsupialia 

 present important variations from the normal mammalian 

 arrangements. For a description of them we refer the reader 

 to the systematic account of these orders. 



Much might be written about the form of teeth. We must 

 content ourselves with the following remarks. The incisors are 

 generally chisel-shaped, with a cutting edge ; the canines conical 

 and pointed ; the premolars compressed and either conical or 

 cutting ; the molars with broad crowns and crushing. But 

 the posterior premolars frequently resemble or approximate to 

 the molars in form. There is frequently a ridge round the base 

 of the crown just above the neck called the cingulum. The crown 

 of the premolars is frequently elongated ante ro -posteriorly and 

 provided with smaller accessory cusps, as a rule one on each side 

 of the main cusp or tubercle. Such a three-cusped tooth with 

 the cusps all in one plane is called triconodont. Sometimes the 

 three cusps are not arranged in a line, but in a triangle, the teeth 

 of the upper jaw having one inner and two outer cusps ; those 

 of the lower jaw having two inner and one outer cusp. This 

 form is called the tritubercular (sometimes trigonodont) : it is 

 more commonly found amongst the molars than the premolars. 

 It is frequently complicated by the addition of a posterior heel 

 or ialon (Fig. 283) which may have one or two cusps. Such a 

 form of tooth which is usually in the molar series is called tuber- 

 cular -sectorial. Further additional cusps may be developed be- 

 tween the three primary cusps. Most commonly there is one 

 such additional cusp, so that if the tooth has no talon it becomes 

 quadricuspidate, the cusps or tubercles being arranged in an 

 anterior and in a posterior pair. To these four cusps one or 

 two may be added ; if one, it may be placed between the 

 cusps of the posterior pair, or between those of the ante- 

 rior pair ; if two, one of them is between the cusps of the 

 anterior and the other between the cusps of the posterior pair. 



