538 MR. W. L. SCLATER ON SOME INDIAN MVRIVM. [JuTlC 17, 



internal, one posterior cusp, and in addition has externally a slight 

 projection which seems to represent a rudimentary external cusp. 



The molars of the lower jaw are even more remarkable: the 

 anterior one consists of eight cusps arranged exactly as those of 

 the molar of the upper jaw, but the anterior external cusp is 

 wanting ; the second lower molar consists of six cusps arranged as in 

 the corresponding tooth above, while the posterior molar consists of 

 four cusps only, representing the two central and two external cusps, 

 with a slight trace of the anterior external cusp. 



This curious dentition, taken in conjunction with the facts above 

 mentioned about the dentition of Cliiropodomys, seems to lead to 

 the inference that we have in Hnpalomys a very primitive form of 

 Eat, by which the dentition of Mus maj' to a certain extent be 

 explained. The upper molars of Mus are easily derivable from a 

 molar such as that of Ilcrpaloinys by the increase in size of the 

 central row of cusps and the suppression of one of the lateral 

 ones ; so, too, with Vandeleuria. In the case of the molars of the 

 lower jaw the condition in Mus and Vanddeuria seems to be directly 

 connected with that in Hapalomys by the in<^ermediate condition as 

 exhibited in CMropochmys ; in Hapidomys the external row of 

 cusps is ah-eady slightly inferior in development to the central and 

 internal rows, and in the case of the anterior molar one cusp is 

 already lost ; in Chiropodomys the outer row of cusps has been 

 reduced to the cingulum running along the external face of the 

 molars described above : while in Vandeleuria and Mus the outer 

 cusps have altogether disappeared, not leaving any trace even of 

 the cingulum, and the molar is bilaterally symmetrical. 



These remarks are merely suggested as a method by which a 

 small step in the evolution of Murine molars may have taken place; 

 it would be absurd to lay any great stress on this theory, as I have 

 only been able to examine the skulls of Indian Eats, and I do not 

 claim to have any acquaintance with the numerous exotic forms. 



The following are the measurements of the unique spirit-specimen 

 of Hapalomys longicavdatus : — Head and body 5'27, tail 7'9, hind 

 foot 1-05, forearm and hand 1-5S, ear-conch "35, muzzle to ear 

 (skull extracted) I'-iO. 



List of the Types of Muridaj in the Indian Museum. 



Eemarks. 



