164 Mr. M. A. C. Hiiiton on the 



are given in the following paragraphs. I would hasten to say, 

 however, that although this homologization appears highly 

 probable to me (and, I am permitted to say, not improbable 

 to Mr. Thomas) J both my colleague and I are strongly of 



opinion that the conventional formula m. ^ '^'g should be 



adhered to, at least in ordinary systematic writing, until such 

 time as definite proof of the " mjJ. 4 theory ^^ can be sup- 

 plied. Great difficulty and inconvenience arise in systematic 

 work from too readily following unproved theories of tooth- 

 homology*; and in the present instance, if it turn out that 

 so important a change in the dental nomenclature of this 

 great family becomes necessary, then the innovation should 

 be made in a formal and conspicuous manner and not be 

 merely indicated by eccentricities in the lettering of figures 

 illustrating the teeth of some small local group. 



The argument in support of the '-'mp. 4 theorj^" may be 

 shortly stated as follows: — In many Muridse (and particu- 

 larly among Microtinae) the lower front cheek-tooth is 

 extremely complex ; to a lesser extent this is also true some- 

 times of its opponent in the maxilla (e. g., Dicrostonyx, 

 Eothenomys^ Chtruromys, and Apodemus epi?nelas) , Wherever 

 the available material permits us to work out the morpho- 

 logical history of the cheek-teeth, we find that within the 

 Muridye (and in nearly all the other rodents) a more complex 

 condition of the crown is a primitive character as opposed 

 to a more simple condition, which invariably appears to be 

 the result of specialization. An enormous mass of facts 

 might be cited in support of this statement, but it will 

 suffice to say that clear evidence of the direction of dental 

 progress, from the complex to the simple, in this family 

 is afforded, firstly, by those very rare cases in which it is 

 possible to trace a single phylum forwards in time (e. g., 

 Mimomys in successive horizons of the Upper Pliocene and 

 Lower Pleistocene of Britain) ; secondly, by the phenomena 

 manifesting a progressive reduction and simplification of the 

 cheek-teeth in the individuals of many j-pecies (e. ^.^ Arvicola 

 amphibius) as they pass from extreme youth to maturity ; 

 and, lastly, by many living and extinct genera in which the 

 species seem to show a series of gradations from more 

 primitive forms, with relatively complex teeth, to more 

 specialized forms, with a simpler type of dentition. In the 

 last class of case the personal equation is no doubt active ; 

 but the observer who has had training in dealing with cases 



* Cf. Thomas, Ann. & xMag\ Nat. Hist. (9) ix. p. 071, footnote (1922). 



