100 MICROTIN^ 



sometimes recurved or " opisthodont." Such strongly curved 

 upper incisors are better adapted mechanically to resist the power- 

 ful thrust of the lower incisors, by which the chief part of the work 

 of gnawing is performed. Where the upper incisors have become 

 more or less straightened and protruding {i.e., " proodont "), 

 forming smaller segments of larger circles, their possessor has 

 abandoned to a greater or less extent the gnawing habit ; such 

 " proodont " incisors are used sometimes as forceps, for picking 

 out small seeds of grasses, etc. ; but more often proodonty is CHie 

 of the chief expressions of fossorial specialization, the proodont 

 species using the upper incisors for digging burrows. 



Some very interesting correlations of minor features with the 

 curvature of the upper incisors are worthy of note. Many facts, 

 observable in such widely different orders as the Primates (Man 

 himself, amongst others), Chiroptera, Insectivora, Carnivora, 

 Ungulata, and Marsupialia, as well as in Rodentia, make it prob- 

 able that the incisor teeth of mammals were originally multi- 

 cuspidate or multitubercular. The advanced position of these 

 teeth in the jaws, far in front of the point where the jaw muscles 

 can drive complex teeth with advantage, together with the mam- 

 malian need for anterior trenchant teeth, has led to their simplifi- 

 cation ; but traces of an ancient primitive complexity are to be 

 seen, either as normal features or as atavistic variations, in all the 

 orders above named. 



In rodents traces of coronal tubercles and of the valleys 

 between them may be seen sometimes on the tips of quite unworn 

 incisors; the unworn incisor of Trogontherium is an example.^ 

 Usually such features are quite ephemeral ; but in certain groups 

 (e.g., Lagomorpha), or certain genera (e.g., Synaptomys, Proe- 

 dromys, and Prometheomys among Microtinse) grooves which may 

 be regarded as the external vestiges of the primitive valleys of the 

 incisor occur, usually on the anterior heavily enamelled face of 

 the tooth, but sometimes, though less clearly marked, upon its 

 weaker posterior surface as well. In many genera (e.g., Lemmus 

 and Myopus) in which the upper incisors cannot be described as 

 grooved, feeble traces of such a structure can be seen by reflected 

 light on holding the tooth at an appropriate angle. In the molars 

 of certain voles {e.g., the 7n^ of Mimomys) the true significance of 

 such grooves is shown most clearly (see p. 115); wherever it 

 occurs either in the incisors or in the molars, such a groove 

 may be taken to represent the mouth of a primitive valley of 

 the tooth-crown; usually all other trace of the valley (except 

 at the summits of quite unworn teeth) has vanished, but at 

 the mouth a mere notch in the periphery of the tooth has stamped 

 itself upon the enamel organ and dentinal pulp of the ever- 

 growing incisor, and so persists, partaking in the general progress 

 towards hypsodonty, though it is often, or usually, quite devoid of 

 functional importance. In so far as it represents the last, though 

 1 HiNTOX, Ann. Mag. N.H., [5], 8, p. 189, 1914. 



