1897.] MAX.A.GAST GEXrS BRACHTtJROMYS. 709 



the former are perfectly similar to the following tooth and to those 

 of B. ramiroMtra, even in the obUque direction of the transverse 

 ridges : whereas in Mhizomys these are placed at right angles to the 

 long axisof the skull. As in B. ramiroMtra, m. ?) sup. of Tachyoryctes 

 is longer than the two anterior, this being a consequence of a 

 fuller development of its posterior portion ; that is, 3 remains longer 

 independent from 5. 



The different proportional size between the molars of Tachy- 

 oryctes and those of Mhizomys is apparently to a great extent due to 

 the influence exercised by the incisors. In old specimens of Tachyo- 

 ryctes (Plate XXXIX. tig. 6 a) the upper molars increase in size 

 from before backwards. The upper incisors in this genus form a 

 smaller segment of a circle than in Jihizomys, so that their posterior 

 end interferes with the development of m. 1. In the latter genus 

 the segment is a larger one ; as a consequence, the first molar is 

 partly or totally (according to the different species) -n-ithdrawri 

 from the influence exercised by the incisor, which interferes with 

 \.\\^. posterior molars. 



In Rh. badlus the hinder end of the incisor reaches farther 

 backwards than in any other species ; ra. 1, which is from the 

 beginning the largest of the three (Plate XL. figs. 1 a, 2 a), 

 remains such to an advanced stage of wear (Plate XL. fig. 3 a) ; 

 the two posterior molars soon diminish in size. This decrease 

 in size, from m. 1 to m. 3, is more intensified with age.- — In 

 Rh. sumatrensis, the incisor does not seem to affect the molars, 

 their proportional size remaining the same in youth and advnnced 

 age (Plate XL. figs. 5 a, 6 a); m. 3 sup. is very small from the 

 beginning. — In Rh. pminosus and Rh. sinensis (Plate XL. fig. 7 «) 

 the incisor forms a smaller segment of a circle than in sumatrensis, 

 and a much smaller one than in hadius. I have not all the stages 

 of wear of R7i. sinensis, but there seems to be no essential difference 

 from Rh. pruinosus. In this latter the unworn m. 1 sup. is eon- 

 sidernbly elongate (Plate XL. fig. 4 a) ; very soon it becomes 

 reduced to the size of m. 2, and in very old dentitions it is even 

 smaller than the latter. 



To recapitulate the foregoing with regard to the two extremes, 

 Rhizomys hadius and Tachyoryctes. In both, m. 1 sup. is, before wear, 

 slightly larger and especially longer than m. 2. In old specimens 

 of Rh. hadius both teeth, unaffected by the incisor, maintain 

 their relative proportions, whereas both become considerably 

 reduced in Tachyoryctes. The latter "s ra. 1, which is the most 

 interfered with by the incisor, is eventually reduced to the size 

 of m. 2, and in old individuals becomes even somewhat smaller ; 

 m. 1 becomes equally much reduced in size. In Tachyoryctes the 

 greater vei'tical extension of the molars is a further reason for their 

 being very scon interfered with by the incisor. M. 3, which from 

 the beginning is somewhat atrophied in Rh. hadius, becomes more 

 so with advance of wear ; whilst in Tachyoryctes, where the posterior 

 part of m. 3 is on the contrary somewhat produced, as compared 

 with its anterior molars, and with m. 3 of Rh. hadius, this tooth 

 maintains its proportions even in old individuals. 



