MURINE 499 



This system is based upon the work of Winge, Fleischmann, 

 Osborn, Forsyth Major, and Hinton^; it is illustrated in 

 Plate XXVIII., in which cheek-teeth of Cricetims, Microtince, 

 and Murines are comparatively represented. 



In upper cheek-teeth the tubercles of the median row 

 (x, y, z) are usually conspicuously larger than those of the 

 outer or inner rows ; those of the inner row have their axes 

 more nearly vertical than have those of the outer and median 

 rows, and in the various sub-families and individual genera of 

 Muridce this row of tubercles has suffered numerical reduction 

 to a more marked extent than have the others. In Apodemus, 

 for example, ni^ has three inner tubercles (PI. XXVIII., Fig. 4, 

 ji/, 6, 7) ; in Epimys this tooth has two, cusp 7 being lost ; and 

 in Dendromyines — an African group — only cusp 6 remains. The 



' Winge {Vid. Med. Nat. For. Kjob., 1882, 15) came to the conclusion that 

 three cusps, numbered by him from before backwards, i, 2, and 3, which are 

 prominently developed upon the outer sides of upper and the inner sides of lower 

 molars in some Marsupials, Insectivora, and Chiroptera, are the most ancient 

 elements of the mammalian molar ; he identified their homologues, or worked out 

 their fate, in the teeth of other mammalian orders. Two other cusps, internal in 

 upper, external in lower teeth to cusps i, 2, and 3, were regarded by Winge as later 

 additions, and were numbered as 4 and 5. Here, according to Winge, the develop- 

 ment of the lower molars stopped ; but in the upper teeth, internally to cusps 4 and 

 5, two new ones, 6 and 7, successively appeared. In his great papers on the 

 Lagomorpha (Trans. Linn. Soc, Zool. [2], vii., 433, Nov. 1899), and on the genus 

 Brachyuromys {Proc. Zool. Soc, 1897, 695), Forsyth Major adopted Winge's notation 

 for the cusps of upper molars ; but he recognised the fact that the evolution of the 

 lower molars is in a more and not a less advanced stage than is that of the upper 

 teeth— a result confirmed by Stehlin's researches upon the dentition of the pigs 

 {Abhand. Schweiz. Palaont. Gesellsch., xxvi., 22 et seq., 1899). Forsyth Major was 

 therefore able to identify in lower molars the homologues of the cusps 6 and 7 of the 

 upper ones ; in addition he took into account some other elements not recognised by 

 Winge, viz., the " intermediate " tubercles. 



It has long been known that the inner and outer sides of upper molars correspond 

 respectively with the outer and inner sides of the lower teeth ; Fleischmann 

 [Sitzungsber. Preuss. Akad. Wiss. Berlin, 1891, ii., 891) went a step further and 

 asserted that the anterior and posterior ends of upper molars are respectively 

 homologous with the posterior and anterior ends of the lower cheek-teeth ; a lower 

 molar is therefore a completely! inverted image of an upper one. This view was 

 contested by Osborn {Bull. Amer. Mus. Nat. Hist., 1892, 84), but it has been 

 endorsed by Forsyth Major {ProC' Zool. Soc, 1893, 201), and it is supported by 

 Hinton's work. Forsyth Major thought that the cheek-teeth of rodents were derived 

 by a process of simplification or reduction from a multituberculate prototype, and in 

 this he is followed by von Mdhely and Hinton. The latter, in his system of cusp 

 notation, takes notice of some ancient elements of the rodent molar which, hitherto, 

 have escaped recognition, and gives effect to the results of all the work briefly 

 reviewed above. 



