122 MICROTIN^ 



" always present a longitudinally biserial arrangement and never 

 develop a functional third series on lingual side of crown " ; whereas 

 in Muridse, as they define the family, the upper teeth have " a 

 functional row of tubercles on lingual side of crown internal to the 

 protocone and hypocone, these tubercles entering conspicuously 

 into the plan of modification of the crowns." This classification 

 is based, in my opinion, on a misconception ; all three rows of 

 tubercles are j^resent in Cricetinse, Nesomyinaj, and Microtinse as 

 well as in the " Muridse " of Miller and Gidley. The inner series 

 (cusps 6 and 7, etc.) are present in all ; it is the median row and 

 not the inner row that fluctuates in development. The median 

 tubercles, as we have seen, are well developed in primitive 

 Nesomyinae ; and in their transformed, modernized state they 

 contribute largely to the crown of the Microtine molar. But in 

 Cricetinse, as in the still older Cricetodonts, the median tubercles 

 sufEer reduction and eventually disappear. In the beautiful 

 figures of South American forms, published long ago by Hensel,^ 

 clear traces of these median tubercles are visible (cf. Hensel's 

 figs. 23a and b, 25a and b, and 26a and b) ; and in unworn or but 

 very slightly worn upper molars of Neotoma small vestiges of these 

 median tubercles occur. 



In Murinse the median tubercles are excessively developed, 

 more or less at the expense of their neighbours, as was recognized 

 by Osborn.^ The cusps which Miller and Gidley refer to as 

 " protocone " and " hypocone," in the passage quoted, are in 

 reality the " protoconule " and " metaconule," cusps y and z of 

 the notation used by me. Clear trace of such an excessive develop- 

 ment of the median row, transformed as it is, is met with in many 

 Microtines, as for example, in the unworn m^ of Ondatra described 

 above. Among Murinse forms like Apodemus epimelas and 

 Chiruromys {Pogononiys) present the most primitive, least- 

 reduced molars met with in the subfamily ; but in all, the longi- 

 tudinal simplification of the molar crowns has proceeded further 

 than in the lowest Microtinse. 



The enamel in the unworn teeth of some of the more highly 

 specialized Murinse, as was first pointed out by Hensel,^ does not 

 extend over the summits of the tubercles. In Microtinse a similar 

 condition is frequently seen in unworn teeth. Atrophy of apical 

 enamel is clearly a specialization effected in order to render the 

 teeth serviceable for the particular task before them from the first 

 moment that they cut the gum, a very important adaptation in such 

 precocious animals as young mice. 



Summing up, there is in my opinion ample evidence to prove 

 that all the Muridse (and indeed all the Simplicidentate Rodents) 



1 Hensel, Abhandl. Konigl. Akad. Wiss., Berlin, 1872, Taf. i-iii. 

 ' OsBORN, " The Rise of the Mammalia in North America," Address 

 Boston, 1893, p. 19. 



^ Hensel, Zeitschr. deutsch. geol. Ges., 8, p. 283, 1856. 

 ScHLOSSER, Die Nager des europaischen Tertiars, p. Ill, 1884. 



