INTRODUCTION. 



more pronounced, and may be found on one side 

 only or on both at once. We then get teeth which, 

 like those of the rodents, exhibit folds or grooves 

 on the worn surfaces. These folds gradually become 

 in different animals more sinuous and deeper, till 

 at last they meet in the interior, and give the teeth 

 the appearance of being composed of two halves, or 

 of two columns set together on the grinding surface 

 of the tooth like half-moonshaped islets, and making 

 it appear as if the teeth had been subjected to a 

 certain amount of pressure. All these different and 

 variable forms are nevertheless deducible in the 

 end from a crown with sharp or blunted tubercles; 

 and the oldest placental mammals exhibit such 

 types with varying characters, types which we now 

 designate as insectivorous or omnivorous teeth, and 

 which may develop further in the one direction or 

 the other. Palaeontologists have made the observa- 

 tion that the folds of the teeth are originally re- 

 markably simple, but always become gradually 

 more complicated in their descendants. The series 

 of teeth observed in the development of horses and 

 ruminants afford excellent examples of this in- 

 creasing complicacy. 



The forms with deep folds dividing up the teeth 

 may be combined with others which appear to have 

 arisen from the fusion of several small teeth. The 

 molars of the elephants present the most complete 

 example of this type, in which tooth-fragments are 

 agglutinated together by cement into a single large 

 tooth. On the other hand, it would be possible to 

 maintain, if we traced the series of teeth from the 

 mastodons downwards through the extinct ele- 

 phants, that these tooth-fragments, each of which 

 has a separate root, have arisen from a continued 

 process of division. 



But, however that may be, the structure of the 

 teeth and the arrangement of the different kinds 

 afford the surest marks for distinguishing affinities 

 between the different mammalian types. 



Usually the teeth are divided in accordance with 

 their position in the jaws, but with reference also 

 to their forms and the relations brought to light 

 when the milk-teeth give place to the permanent 

 set. We will now give the explanation of the term.s 

 made use of in describing the dentition. 



The upper teeth are set in two pairs of bones, 

 the premaxilla; in front and the maxillae behind. 

 All the teeth set in the premaxillae, whatever be 

 their form, are called incisors. The tusk of the 

 elephant, as well as the curved, sharp-edged, chisel- 

 shaped tooth of the rodent, the pyramidal tooth 



of the musquash or North yXmerican musk-rat 

 {Fiber rjibctliinus), and the recurved hook-like tooth 

 of the shrews, are all incisors, for they are set in the 

 premaxilla, the separation of which from the maxilla 

 remains almost always visible in the mammals. 

 In the lower jaw, which is composed of a single 

 bone on each side, the mandible, there is not the 

 same valuable criterion for distinguishing the in- 

 cisors as in the upper jaw; and in it all those 

 front teeth which present a greater or less resem- 

 blance to the incisors of the upper ja\\' are generall)' 

 distinguished by the same name. 



After the incisors there often comes a recurved 

 tooth which in most cases rises above the level of 

 the others. This, which is especiall)' characteristic 

 of the carnivores, has been called the canine. But 

 it is found also in omnivorous animals and even in 

 certain herbivorous ones, for instance, in the musk- 

 deer, and is especially large and powerful in the 

 male, in which it is a weapon not to be despised. 

 In the rodents, in most of the ruminants, and in fact 

 in many mammals, it is wanting altogether, or is 

 found only in the upper jaw. 



Behind the canine is developed the series of 

 molars, which are divided into premolars and 

 true molars. The only means which we have for 

 distinguishing these two groups from each other 

 is that afforded by the exchange of teeth. The 

 term premolars is applied to all those which are 

 shed and get replaced by others, while those \\'hich 

 belong solely to the permanent dentition are 

 designated true molars. But in practice it is often 

 difficult to carry out the distinction from this point 

 of view, since the milk dentition is not alwa)'s 

 known. In that case the first four of the row of 

 teeth situated immediately behind the canines are 

 commonly known as premolars, being mostly 

 distinguished from the succeeding ones by their 

 simpler form. 



In order to express the distinctions presented by 

 these teeth naturalists have devised certain formulae, 

 of which we have made use in this wor]< also, and 

 which we must therefore explain. Different authors 

 have naturally adopted different formula;, but we 

 have employed those which appear to us the 

 simplest. 



Dental Formulae. — The teeth are always arranged 

 symmetrically; on the right side there is alwa)-s the 

 same number of teeth as on the left. It is conse- 

 quently sufficient to give those on a single side. 

 On the other hand, the number of the teeth in the 



upper jaw is often considerabh' different from that 



2 



