132 MR. O. THOMAS ON A NEW GENUS OF MURID/E. [Feb. 21, 



molar series 5-6, of m' 3*5, of m'^ 1'9, of m^ 10; lower jaw, length 

 (bone only) 20*0, (to incisor tips) 22*0 ; vertical diameter of ramus 

 below m^ 3'0, just behind symphysis l"/ ; angle to tip of corouoid 

 process 8"0 ; length of lower molar series 5'6. 



Hub. Lower Congo {M. Petit). 



The very special and unusual interest that this new genus possesses 

 lies in the fact that it represents a " missing link " in the phylogeny 

 of the Muridse, as it is intermediate between the two great groups 

 of that family, the Mures and Criceti \ The distinction between 

 these two groups is, broadly, that the upper molars of the latter 

 have their cusps arranged biserially, while in those of the former 

 they are triserially placed. (Plate V. figs. 9 and 10.) 



heomys thei'efore (Plate V. fig. 7), with its hicuspidate anterior and 

 tricuspidate middle lamina of ^, shows an intermediate condition 

 between the two, and probably represents an early stage in the evo- 

 lution of a triserially from a biserially arranged dentiiion. That it is 

 not a later specialization of the Murine group through the loss of 

 the cusp A 3 is shown by the very primitive characters present both 

 in i_w^ and in the formation of the infraorbital foramen. In the 

 complete systematic arrangement of the Muridae, therefore, we shall 

 have to look upon Deomi/s as forming by itself a special section, the 

 Deom>/es, intermediate between the Mures and Criceti. 



From the distribution of the two latter groups, and the characters 

 of their fossil allies, it has long been recognized by students of the 

 subject that the Criceti, with their comparatively simple teeth, repre- 

 sent the original Muridae, once spread over nearly the whole world ^ 

 but now, owing to the competition of the more highly specialized 

 Mures, almost confined to America and Madagascar, in each of 

 which places they still form the only Muridse. It has frequently 

 been stated or assumed that they are entirely confined to these two 

 j)arts of the globe, and on this assumption, without reference to the 

 palseontological history of the group, great and altogether dispro- 

 ])ortionate stress has been laid upon their distribution as affording 

 evidence of the more or less direct connection of the American and 

 Madagascar faunae^. As a matter of fact, no instance can better 

 support Mr. Wallace's views'* on the derivation, and especially on the 

 undoubtedly American relationships, of the Madagascar fauna, since 

 the three conditions on which his views are based — viz.: (l)the 

 lowly nature and therefore considerable antiquity of the Madagascar 

 forms, (2) their forner wider distribution, and (3) their powerlessness 

 to resist the competition of rival forms now paramount in Africa — 

 are all conspicuously present in the Criceti, the group to which the 



' This group has beeu called tlie " Sigmodoutes " by most English aud 

 American authors ; but that name should give way to " Criceti " for reasons 

 shown below. See also Winge, Vid. Medd. 1881, pp. 25 and 54; E Museo 

 Lundii, iii. p. 109 (1887). 



- There is as yet no evidence of the former presence of any Cricetine form in 

 the Austi-alian region. 



^ Cf., for example, Kolbe (SB. Nat. Freund. 1>*87, p. 147), whose remarks 

 are l)asod on Potei's's account of Nesomi/s {op. elf. 1870, p. 54). 



* ' Islam! Life.' p. 383 ct srqq. (1880). 



