EVOLUTION OF INCISORS 99 



portions extending backwards between the molars to 

 terminate near the palatal surface of the hinder part of 

 the maxilla opposite the hinder end of m'. 



Cheek-teeth essentially as in Prometheomys, biit m' 

 with its first outer infold still more reduced ; m^ with 

 the first inner infold less reduced ; m^ in some species 

 less reduced, the fourth and fifth triangles in such 

 forms being more clearly separated from the anterior 

 loop. 



Fur fine, short, dense, and mole-like ; eyes small ; ears 

 vestigial and concealed; hands and feet of moderate 

 size; claws very small ; palms and soles naked ; plantar 

 pads 6. Tail very short, with a long terminal pencil. 

 Mammae 2 — 2 = 8 Ellobius. 



/. Special Notes. 

 1. The Evolution of the Incisor Teeth. 



Gnawing, as is well known, is the most characteristic of all 

 rodent habits. It is the habit which from the first has exerted 

 the greatest and most continuous influence upon the structure 

 of these mammals. By degrees it has produced all those essential 

 modifications of the incisor teeth, of the jaws, of the jaw muscles, 

 and of the bones to which those muscles are attached, which 

 now distinguish the order Rodentia so sharply from all 

 others. But, if this be so, gnawing must be regarded as the 

 primitive rodent habit, and departures from that habit, whether 

 great or small, are specializations, progressive or retrograde accord- 

 ing to the point of view. In turn this gives rise to the reflection 

 that incisor teeth, that by their form and curvature are perfectly 

 adapted for gnawing are, among living rodents, more primitive 

 than those which have lost something of this form and curvature 

 in becoming adapted to other uses.^ 



In rodents with typical gnawing propensities the exact form 

 and curvature of the incisors, no doubt, depend very largely upon 

 the nature of the substance to be gnawn ; the hard shells of nuts, 

 the trunks of trees, and the stems of grasses make very different 

 demands upon the teeth. But in all such typical rodents the 

 upper incisors are strongly curved, " orthodont " ^ at least, and 



* Lest this view appear far-fetched I will add that no one looking at the 

 essential characters of a rodent incisor, namely, its life-long growth and 

 the restricted distribution of its thick enamel, can doubt that it is primarily 

 the product of the gnawing habit. All rodents, whatever their present 

 habits, have descended from ancestors that gnawed vigorously. A good 

 parallel to some of the retrogressive specializations observable among 

 Rodentia is afforded among Camivora by Proteles; no one can fail to 

 recognize the stamp of its predatory ancestors in its incisors and canines ; 

 but no one would attribute predatory habits to Proteles itself after inspection 

 of its vestigial cheek-teeth. 



« Thomas, Ann. Mag. N.H., [9], 1, p. 35, 1918 ; ibi<I., [9], 4, p. 289, 1919. 



