BRITISH FOSSIL SPECIES 125 



dm\- m|f|, which iu the more usual notation would be expressed as 

 mpl, mY?2- 



Kellogg ^ used the formula p} nij^. In his paper he cites the 

 views of Forsyth Major and myself, but does not indicate clearly 

 whether he intends to agree with us or not, although he seems 

 to admit that the homologization of the front lower cheek-tooth 

 in Microtinye with mp^ would afford an explanation of its 

 complexity. 



The reasons for the suggested homologization of the teeth in 

 question with vip^ have been given fairly fully by me.^ Briefly 

 they are that the lower tooth is so complex in Microtinse that we 

 are unable to derive it from any known rodent molar without 

 postulating the addition of new parts ; that there is no evidence 

 of increasing complication at all in this tooth ; and that every- 

 where, even when most complex, the tooth is undergoing reduc- 

 tion whether we trace it forwards from youth to age in one species 

 or forwards from one species to another in successive geological 

 horizons. Posterior milk-molars are commonly more complex 

 than true molars ; therefore the tooth in question may be a 

 persistent milk-molar. The facts (1) that in Mus musculus the 

 last molars are sometimes suppressed (in South America *), 

 indicating that numerical reduction, when it occurs, takes place 

 from behind ; and (2) that in those rare cases in which an extra 

 tooth is developed in Muridse it appears at the posterior end of 

 the series (Microtus ag rest is as recorded by Wiuge * ; Saccostomus 

 hildcB, B.M.^), so that according to the current notation the extra 

 tooth would have to be regarded as mi ; are cited in support of 

 this view. 



If, however, the view now maintained that the molars of all 

 Simplicidentata were primitively as complex at least as they are 

 in the most primitive Microtina3 be correct, the argument above, in 

 so far as it is based upon the complexity of the anterior lower 

 tooth, loses its force. In any case I can only reiterate the opinion 

 expressed iu 1923 that no change in the commonly accepted 

 notation of the cheek-teeth of Muridaj should as yet be adopted. 



4. The Range in Time of the British Fossil Microlince. 



Fossil remains of Microtina3 occur abundantly in the later 

 Pliocene and Pleistocene deposits of Britain. A study of the 

 remains collected from the deposits of the Norfolk coast and from 

 the terrace-deposits of the Thames shows that the individual 

 species have short ranges iu time. They thus afford help to the 

 geologist endeavouring to correlate scattered or isolated cavern 



* Kellogg, Univ. Cal. Publ. Zool., 21, p. 245, 1922. 

 ' HiNTON, Ann. Mag. N.H., [9], 11, p. 162, 1923. 



* WiNGE, Jordfundne og nulevende Gnavere fra Lagoa Santa, p. 601. 



* WiNGE, Vidensk. Meddel. naturh. Foren. Kjobenhavn, 1881, 1882, 

 p. 24 (footnote). 



» Schwann, P.Z.S., 1906, p. 110. 



