EVOLUTION OF INCISORS 101 



highly modified, vestige of the primitive complication of the incisor 

 crown, the presence of a groove may be regarded as primitive, 

 and the absence of such a groove as a specialized feature of the 

 rodent incisor. It is interesting to note that grooved incisors are 

 always strongly curved incisors ; proodont incisors with grooves 

 are unknown to me among Myomorpha. On the other hand, 

 strongly curved incisors may show no trace of grooves ; so that 

 it would seem that the minor primitive feature (a groove) dis- 

 appears before the major primitive feature (strong curvature). 



Another minor character, very widely distributed among 

 rodents, seems also to be directly correlated, as a general rule, 

 with the more primitive strong curvature of the upper incisors. 

 This is the pigmentation of the enamel, the colour of which, from 

 genus to genus, ranges from deep reddish brown to very pale 

 yellow and even to pure white; the significance of the 

 colour when present, and of the similar pigmentation of enamel 

 seen in the red-toothed shrews, is not known. Its wide distribu- 

 tion in all families of Simplicidentate Rodentia, its high antiquity, 

 dating at least back to Oligocene times, the circumstances that the 

 most strongly curved incisors are the most deeply pigmented and 

 that highly specialized " proodont " incisors are almost invariably 

 pure white, or if tinted are always very pale, are facts which 

 lead me to believe that pigmentation of the enamel is, as opposed 

 to non-pigmentation, a primitive character. 



Many years ago Ryder ^ pointed out that when the incisors 

 are wider than thick, the gnawing habit is feebly developed {e.g., 

 Microtinse), and that when the incisors are thicker than wdde, 

 the gnawing habit is greatly developed {e.g., Murinse). In Micro- 

 tinae and in many other rodents the upper incisors, as they become 

 straighter and more proodont, become rounder in section and less 

 truly chisel-like towards the cutting edge, a further proof, if one 

 were needed, that in the group under consideration " proodonty " 

 is the result of specialization for habits inconsistent with ordinary 

 gnawing. 



The use of the incisors as digging instruments imposes upon 

 them the burden of unusually rapid waste; to repair this their 

 pulps, in response to the stimulus of extra work, have become 

 larger and more active, pushing their way back to points in the 

 jaws considerably behind those reached by the pulp cavities of 

 the incisors of genera that are less fossorial. In Ellobms, the most 

 specialized of the Microtina; in this respect, the upper incisor has 

 pushed back between the molars to the hinder border of the maxilla, 

 often causing a fenestration in the palatal surface of this bone ; 

 and the lower incisor terminates behind level with the condyle. 

 In all other Microtine genera the up2)er tooth terminates 

 posteriorly in front of m^, the lower well below the condyle. 



» Ryder. Proc. Acad. Nat. So. Philadelphia, 1877, p. 314. 



