EVOLUTION OF MOLARS 119 



At this point, before proceeding to a more general comparison, 

 it will be useful to sum up the results obtained from a study of the 

 molars of the Microtina; alone. 



Firstly, there is evidence that the cheek-teeth above and below 

 have been evolved from teeth in which there were three longitu- 

 dinal rows of tubercles. Of these rows the median has retained, in 

 its modernized form, its full functional importance ; but the 

 outer row in the lower and the inner row in the upper molars have 

 suffered reduction. 



Secondly, there is evidence that all the molars have to a greater 

 or less extent suffered reduction in a longitudinal direction. 

 This reduction has affected the ends of the teeth ; it has been most 

 far-reaching in m'f and )ii^; the anterior ends of m\ and the 

 posterior end of m^, free from contact with neighbouring teeth, 

 have suffered least. Apart from cusp x^, normally lost by all the 

 teeth except m^, reduction has chiefly affected the posterior ends of 

 the upper and the anterior ends of the lower molars ; in other words 

 simplification has proceeded from behind forwards in upper molars, 

 from before backwards in lower molars. The parts, which accord- 

 ing to Fleischmann's theory are homologous in the two series are 

 equally liable to reduction above and below ; whereas, as is 

 familiar to all upholders of the tritubercular theory, what are 

 generally supposed to be the homologous cusps have very different 

 fortunes in the two jaws. 



Lastly there is no evidence at all of any progressive complica- 

 tion of the molars within the group. On these various grounds 

 we may reject the theories of the progressive complication of the 

 teeth of Microtinse advanced or supported by many authors.^ 



As the result of my work I conceive the molars of the ancestor 

 of the Microtinaj to have been low-crowned, rooted, multitu- 

 berculate teeth, each tooth, above and below, having three longi- 

 tudinal rows of tubercles. In this primitive form ?/i^, m| and m^ 

 possessed at least twelve, m^ not fewer than fifteen, and m^ from 

 eighteen to twenty-one tubercles. 



We now proceed to a comparison of the cheek-teeth of Micro- 

 tinse with those of other Muridse. The oldest Muridse at present 

 known are the Cricetodonts of early and middle Tertiary age. 

 In the most primitive of these (i.e., those in which the cheek-teeth 

 retain more of the primitive complexity), such as Cricelodon 

 cardurcense Schlosser,^ described from the Miocene of France, the 

 outer sides of the upper molars and the inner sides of the lower 

 molars are nearly as complex as in any Microtinse, showing repre- 

 sentatives of the cusps 1, 4, 2 (never present in Microtinte), 5, and 3 

 with x^ in m^. On the other hand, the median tubercles are much 

 reduced as in modern Cricetinae, and the inner cusps of the upper, 



1 TuLLBERG, Ueber das System der Nagetiere, 1899, pp. 229, 232, 235, 

 23'7. 239, 442-443. 



Miller and Gidley, Joum. Washington Acad. Sci., 8, pp. 436-^37, 1918. 



^ ScHLOSSER, Die Nagcr des europaischen Tertiars, Palaeontographica, 

 31, p. 90, Taf. viii, figs. 28 and 35, 1884. 



