142 



THE GNAWERS OR RODENTS. 



of man or to beavers. Since these incisors 

 go on constantly growing, remarkable mal- 

 formations can be produced in captive animals 

 by preventing them from using these teeth. 

 Only in a single family, that of the hares, do 

 we find behind the large functional incisors a 

 pair of small incisors, which are so placed that 

 they seem to serve as a heel to the former. 1 



The canines are altogether wanting even 

 in the milk-dentition. 



The number of the cheek-teeth is rather 

 small, varying from two to six in each half 

 of each jaw. They resemble each other very 

 closely, so that the premolars cannot be dis- 

 tinguished by their form alone from the true 

 molars. If, however, we trace out their 

 development, we find that there are always 

 three true molars, and that the teeth standing 

 in front of these are liable to be shed. But 

 in certain rodents, for instance, it has been 

 observed that the milk-teeth are shed even 

 before birth, so that these species come into 

 the world with the permanent dentition. 

 The rodents with three molars do not get 

 their single premolar replaced, whether it be 

 shed before or after birth; those with four 

 molars have a single premolar replaced, and 

 thereby present a remarkable point of agree- 

 ment with the marsupials, which also have 

 only one replaceable tooth in their dentition. 

 Lastly, the rodents with five molars have two 

 replaceable premolars. 



The structure of the cheek-teeth, although 

 it varies greatly in details, may be reduced 

 to a few types, which, however, are connected 

 by intermediate forms. We find in the first 

 place rootless constantly-growing cheek-teeth 

 in which the part set in the socket is wide 

 open below, but otherwise resembles the 

 crown in form. Such is the case, for example, 

 with the Capybara. Secondly, we see teeth 

 with clearly-distinguished crown and root, as 

 in the rats. But between these extremes 

 there are transitional forms with a more or 



1 On this account this family is sometimes separated as a distinct 

 sub-order under the name of the Duplicidentata. TR. 



less complete closure of the roots, and we 

 even find genera in which the distinction 

 between root and crown first comes out in 

 advanced age; the root in the young animal 

 is open and gets constricted only with 

 advancing years. 



The relations between the different sub- 

 stances which compose the cheek-teeth, 

 enamel, dentine, and cement (if the last is 

 present at all), vary considerably, and have 

 been made the ground of important zoologi- 

 cal distinctions. In some, for example the 

 octodonts, the cheek-teeth are simple cylin- 

 ders exhibiting a round or oval surface filled 

 with dentine and surrounded with an unbroken 

 ring of enamel. These teeth resemble those 

 of certain edentates. In others again the 

 teeth are tubercled, and accordingly in a 

 certain measure similar to those of the 

 Omnivora or Insectivora. As the tooth gets 

 worn away the tubercles form small isolated 

 patches surrounded by enamel. Such teeth 

 are found in the rats. Vertical enamel folds 

 producing grooves on the outside penetrate 

 more and more deeply into the dentine. If 

 there is, as in the jumping-hares, only one 

 such fold, the worn surface presents a figure 

 like that of a loaf of bread divided by a single 

 groove down the middle. If there are two 

 folds, an outer and an inner, the tooth 

 appears to consist of two halves connected 

 by a bridge, and when these folds curve and 

 wind, as in the beavers, the gnawing surface 

 presents a confused coil of folds, which pro- 

 duces the appearance of a piece of coarse 

 cloth irregularly folded and pressed. Lastly, 

 the folds may, as in the Capybara, pass right 

 across the teeth and thus become subdivided 

 into a number of plates or lamellae connected 

 by cement and having the intervals between 

 them filled with dentine, so that these teeth, 

 which resemble those of elephants, seem to 

 be made up of a number of small teeth pressed 

 close together. There are, accordingly, among 

 the rodents, simple, tubercled, folded, and 

 lamellar teeth. 



