PF.XTAL SYSTEM 



23 



ported on two or more roots. They crush or grind the food, and 

 are hence called "molars." Many animals have, between these 

 two sets, a tooth at each corner of the mouth, longer and more 

 pointed than the others, adapted for tearing or stabbing, or for 

 fixing struggling prey. From the conspicuous development of 

 such teeth in the Carnivora, especially the Dogs, they have received 

 the name of "canines." A dentition with its component parts so 

 differently formed that these distinctive terms are applicable to 

 them is called Heterodont. In most cases, though by no means 

 invariably, animals with Heterodont dentition are also Diphyodont. 



This general arrangement is extremely obvious in a considerable 

 number of mammals; and closer examination shows that, under 

 very great modification in detail, there is a remarkable uniformity 

 of essential characters in the dentition of a large number of 

 members of the class belonging to different orders and not otherwise 

 closety allied ; so much so indeed that it has been possible (chiefly 

 through the researches of Sir Richard Owen) to formulate a common 

 plan of dentition from which the others have been derived by the 

 alteration of some and suppression of other members of the series, 

 and occasionally, but very rarely, by addition. The records of 

 palaeontology fully confirm this view, as by tracing back many 

 groups now widely separated in dental characters we find a 

 gradual approximation to a common type. In this generalised form 

 of mammalian dentition (which is best exemplified in the genera 

 Anoplotherium and Homalodontotherium) the entire number of teeth 

 present is 44, or 11 above and 11 below on each side. Those of 

 each jaw are placed in continuous series without intervals between 

 them ; and, although the anterior teeth are simple and single- 

 rooted, and the posterior teeth complex and with several roots, 

 the transition between the two kinds is gradual. 



In dividing and grouping such teeth for the purpose of descrip- 

 tion and comparison, more definite characters are required than 

 those derived merely from form or function. The first step towards 

 a classification has been made by the observation that the upper 

 jaw is composed of two bones, the j)remaxilla and the maxilla, 

 and that the suture between these bones separates the three 

 anterior teeth from the others. These three teeth, then, which are 

 implanted by their roots in the premaxilla, form a distinct group, 

 to which the name of ".incisor " is applied. This distinction is, 

 however, not so important as it aj)pears at first sight, for, as 

 mentioned when speaking of the development of the teeth, their 

 connection with the bone is only of a secondary nature, and, although 

 it happens conveniently for our purpose that in the great majority 

 of cases the segmentation of the bone coincides with the interspace 

 between the third and fourth tooth of the series, still, when it does 

 not happen to do so, as in the case of the Mole, we must not give 



