Dental Formula of the MurldrG. 169 



Ausf alien veranlassen. Nun stellen aber die !Nagezahiie 

 jiinger Individuen Scgmcnte eines vicl kleiiiern Kreises dcr 

 als die jilterer, so dass sicli also der Na<;ezahn bei zuneli- 

 mendem Alter niit deni od'enen Knde von seiiiem Phitz unter- 

 beziehungsweise oberhalb des d. 1 (inp. 4) nacli hiiiten zu 

 eiitfernen kann. Seiner Wirkuiig mogen wir dann in 

 manchen Fallen die Yerkiimmerung des letzten ^[olaren 

 zuzuscbreiben liaben, wie sic beispielsweise bei Mus vor- 

 kommt. — Icli priitendiere nicht, mit den vorstehendcn 

 Bemerkungeu meine Aiisicht bewiesen, glaube sie aber 

 besser raotiviert zu liaben, als die allgeraein lierrscbende 

 es ist. Die definitive Antwort auf diese und. ahnliche 

 Fragen diirfen wir wohl rait der Zeit von der Palaeontologie 

 erwarten." [' Fossile Pferde/ p. Ill, footnote.) 



Two otlier little facts — trifles in tbemselvesj no doubt — are 

 wortby of note for tbe cumulative effect of tlieir evidence. 

 In Soutli America, as Winge records (and as I bave seen in a 

 specimen from Cordova), tbe atropby of tbe posterior molars 

 in Mus musculus proceeds sometimes to tbe total suppression 



of those teeth ; in Europe m. 3 seem to be constantly present. 



Secondly, in those very rare cases in which an extra tooth 

 is developed in Muridse it appears at the posterior end of 

 the series (Microtus agrestis, Winge; Saccostomus hildce^, 

 B.M.) : tbis extra tooth would have to be m. 4 according to 

 tbe current notation, but it finds a perfectly natural explana- 

 tion as m. 3, sporadically and atavistically returning, 

 according to the mp, 4 theory. 



To conclude, definite proof of tbe bomologization of tbe 



teeth in question with mp, j is not yet forthcoming ; and it 



cannot be expected until palaeontological researcb brings to 

 lijibt really primitive Muridse from some still unknown 

 old Tertiary horizon, possibly in south-eastern Asia. At 

 present tbe Microtina? seem to be tbe group most likely to 

 afford tbe desired information ; but notbing is yet known 

 of the history of that subfamily before the middle or tbe late 

 Pliocene. Of the species then existing some belong to still 

 living genera and are dentally, at all events, as highly 

 specialized as are their living representatives ; others are, in 

 a measure, ])rimitive, but even they have proceeded too far 

 along the path of dental progress to show us more than the 

 direction of that path and its later portions. Such facts 

 indicate clearly bow far we are from the beginning of the 



•* Sclnvaim, P.Z.S. 1906, p. 110. 



