MURINiE 501 



is in complete accord with the results obtained in Microtince, 

 where in addition to cusp 7 other ancient elements {n, n^) are 

 sometimes present though they have not yet been found in the 

 teeth of living Murines. The results in question fully support 

 the theory of a multitubercular origin of the rodent cheek-tooth 

 postulated by Forsyth Major, and are in complete conflict with 

 the tritubercular theory of Copt and Osborn. 



The lower cheek-teeth are in one way more modified than 

 the upper ones ; the outer row of tubercles corresponding to the 

 inner row of the upper teeth is so reduced that it has been 

 either completely ignored or else treated as a mere cingulum 

 by all writers except Tullberg. In Apodemus and many 

 exotic genera this third row is comparatively well-developed, 

 and consists normally of three tubercles in m-^ ; it has, on the 

 other hand, completely vanished from the teeth of Mus and 

 many other genera ; throughout the sub-family it shows the 

 high degree of variability which is characteristic of vanishing 

 structures. From the circumstance that it is a marked feature of 

 the teeth of Apodemus and some other MurineB, that it forms an 

 important part of the teeth of some Microiincs, e.g. Dicrostonyx 

 (PI. XXVIII., Fig. 2), and occurs ephemerally in the young teeth 

 of still more remote relatives, e.g. Spalaxi^^^iy, Species Generis 

 Spalax, 191 3, 305, fig. 10), and from the fact that it comprises such 

 ancient and well-known molar elements as the "proto-" and 

 "hypoconids" of trituberculy, we may conclude that this third 

 or outer row of tubercles was an important feature in the lower 

 molars of the ancestral Muridce. It has suffered reduction and 

 even obliteration in consequence of the hypertrophy of the 

 " intermediate " cusps of the teeth of these remarkable rodents. 



Two other elements of the lower cheek-teeth deserve notice. 

 In »«i and ^2 a posterior "accessory" tubercle (PI. XXVIII., 

 Figs. 4-10) is almost constantly present in Murines; this 

 structure appears to be the homologue of part of the posterior 

 transverse loop of the corresponding teeth in Microtince. In 

 most species of Apodemus, and in some other murines, m^ has 

 an anterior "accessory" tubercle which appears to represent 

 the anterior loop of the m^ of Microtis. 



Because of their forward position in the jaws the greater 

 share of the work of mastication falls upon the anterior cheek- 



