124 MICROTIN^ 



of some other Orders, e.g., the Insectivora, are no doubt more 

 primitive than Rodentia, the latter are in fundamental molar 

 structure among the most primitive of living mammals. In 

 no other group do we find such clear and such generally distributed 

 traces of the original Multituberculate tooth structure. The 

 molar types regarded as primitive by the supporters of the 

 Tritubercular Theory, by Winge, and by the supporters of many 

 other rival theories, are in my opinion secondary and not primitive 

 types, although often very ancient ; these secondary forms have 

 been developed again and again, from Jurassic times onwards, 

 in all Orders as the result of progressive simplification of the 

 primitive Multituberculate tooth. 



Although probably no known Multituberculate can be claimed 

 as ancestral to Rodents, it is interesting to note that some of 

 them, like Polymastodon, developed along lines which are parallel 

 to the direction followed by the Rodentia. It is among Allotheria 

 alone that we find molar teeth which fit the conception of the 

 primitive Simplicidentate molar given above. The earliest 

 Rodents known, those from the Basal Eocene, are already quite 

 highly specialized in many ways, and the Order is evidently one 

 of very great antiquity. 



3. The Dental Formula of the Muridce. 



The question as to whether the anterior cheek-tooth above 

 and below in the Muridse is a true molar (my), a premolar (j5j), or 

 a persistent milk-molar (tojo|) has attracted attention from time 

 to time. 



Owen ^ stated that in rodents " the first or anterior of the 

 molar series, whether the number be 2 — 2, 3 — 3, or 4 — 4, is a 

 premolar ; it has displaced a deciduous predecessor in a vertical 

 direction." But such a replacement is entirely unknown in 

 Muridss, and Owen's formula, pj wirl, lacking the support of any 

 positive evidence, has been abandoned for many years in favour of 

 that now in common use, namely, mj-:o-;f!. 



In 1872 Forsyth Major ^ was led to suspect that the anterior 

 cheek-tooth, above and below, in Muridse is neither a " molar " 

 nor a " premolar " in the ordinary sense of those terms, but is the 

 posterior milk-molar, which in this family has become persistent 

 in each jaw, the permanent premolar (p4), normally replacing 

 this tooth, having been suppressed. Long afterwards, but 

 independently when working at Microtinse, I arrived at a similar 

 conclusion,^ and adopting Hensel's tooth notation (which I 

 have always preferred) both Forsyth Major and I wrote the formula 



1 Owen, Anatomy of Vertebrates, 1868, 3, p. 300. 



2 Forsyth Major, Atti Soc. Ital. Sci. Nat., 15, p. 112 (1872); Palseonto- 

 graphica, 22, p. 75 (1873); Abhandl. Schweiz. palaontol. Ges., 4, p. Ill, 

 footnote 3 (1877). 



3 HiNTON, Proc. Geol. Assoc, 21, p. 490 (1910). 



