﻿JuLv.1896.] SUBGENUS NEOFIBER. 69 



ber the type of a new genus, Anlacomys^^ while M. lyrinciimlis, closely 

 allied to botli M. macropus and M. arvicoloides^ was referred by tlie same 

 author to true AHcrotns.^ This confusion arose from the fact that the 

 subgeneric and generic determinations were based chiefly on dental 

 characters. Hence Microtns macroj^us and M. princifxdis were naturally 

 considered members of the subgenus Microtns, since both have the 

 enamel pattern characteristic of the tetramerodont species of that group. 



The teeth of the type and only known specimen of Microtns arvico- 

 loicles, on the other hand, show certain characters which, although clearly 

 abnormal, led to an entire misunderstanding of the animal's true rela- 

 tionships. The first of these abnormal characters, and the one which 

 suggested the name Avlacomys, is seen in the upper incisors. Each of 

 these has a narrow longitudinal median groove. Thej^can not, however, 

 be considered as entitling the species to generic rank, since similar 

 though fainter grooves are occasionally found in almost any species of 

 Microtns, while they are absent in the vast majority of specimens of 

 'Aulacomys.^ The second abnormality in the type of Microtns arvico- 

 loides is in the form of the front lower molar. This tooth (fig. 35) has 

 two reentrant angles on the outer side of the anterior 

 loop instead of one as usual in Microtns. The supple- 

 mental reentrant angle, like the grooves in the incisors, 

 is purely an individual character, which may crop out ^^^^lormai 



^ 7 ^ i front lower molar 



in any species of Microtns, with the front lower molar oft.v]., siK , inienof 

 formed as in M. arvalis, and which is absent in all the 'Ardaeo)n!i:i^^n-vico- 

 other thirty or more si)ecimens of '■Anlacomys'' that I 

 have seen. The subgenus Aulacomys if retained as distinct from Arvi- 

 cola must rest on characters of enamel pattern alone, since in all other 

 IDeculiarities it agrees i)erfectly with the latter. The difi^'erences in 

 enamel folds are rather considerable, since 'Aulacomys^ has the highly 

 complicated pattern of true Microtns, while the si)ecies of typical Arvi- 

 cola have the simplest pattern of any known. While it seems highly 

 inadvisable to base subgeneric divisions on such characters, the deci- 

 sion rests on purely individual judgment. 



In the Old World numerous ^species and subspecies are probably 

 confused under the name ^Arvicolci anqjh ibins.^ Microtns musignani ( De 

 Selys Longchamps) and M, monticola (De Selys Lougchamps) appear to 

 be especially distinct from Af. terrestris (Linn.). 



Siibgeuus NEOFIBER True. 



1884. Xeofiher True, Science, IV, p. 34, July 11, 1884 (full genus). Type Neofiher alleni 

 True. 



1891. Xeofiher Merriam, North Americau Fauna, No. 5, p. 59, July, 1891 (subgenus). 



Geographical (listrihntion of type species. — Florida. '^Doubtless a com- 

 mon animal in favorable localities throughout the State." (Chapman.) 



1 American Naturalist, XXVIII, p. 182, February. 1894. 



2 American Naturalist, XXIX, p. 940, October, 1895. 



