Rats allied to Oryzomvs and Rliipidomys. 193 



plate little projected forward. Palatal foramiua short and 

 little open. Posterior palate of the general structure of 

 that of Oryzomys and (EcomySj not as in Rhipidomys and 

 Thomasomys. 



Upper incisors approaching the vertical, angle with tooth- 

 row about 80°, deep antero-posteriorly, their front surface 

 flattened and inclined inwards, so that the resulting relations 

 of the two teeth and the shapes of their tips are about as 

 in the Dormice, not as in any of the genera above mentioned. 

 Lower incisors of corresponding form, anteriorly, their roots 

 extended backwaids much beyond the normal, forming a 

 prominent capsule outside the jaw, halfway between the 

 coronoid and the condyle. Both the shape and implant- 

 ation of the incisors therefore indicate unusual gnawing 

 powers. 



Molars showing a remarkable modification of the structure 

 found in CEcomys and Oryzomys, for while the number 

 and positions of the cusps are the same, the various foldings 

 and ridges between and connecting the cusps are almost 

 entirely obsolete. The teeth are therefore almost as in 

 certain Phyllostomid bats, with smooth glossy surface and 

 simple conical cusps, which are evenly spaced, slightly 

 slanted backwards, 6, 4, and 2 in number on the three 

 teeth. Below the teeth are similarly modified, the cusps 

 slanting forwards. 



Genotype; Rhaffomys rufescens {Hesperomys rufescens, 

 Thos.). 



Without a much greater knowledge than I possess of the 

 structural modifications of the molars of this group, and the 

 systematic value that should be attached to their simpli- 

 fication in Rhagomys, I should not venture to express a 

 definite opinion as to its afiinities, but I should suppose it 

 to be, on the whole, most nearly allied to CEcomys, Avith 

 which it agrees in general facies and palatal structure, but 

 from which, as from every other genus of the group, it may 

 be readily distinguished by the remarkable modification of 

 both incisors and molars above described. 



Still younger specimens of Rhagomys will be very welcome 

 to show what trace of the normal foldings and rido-es is 

 exhibited by the molars when absolutely unworn ; but it 

 is evident there cannot b^ much. 



Of the second group, those with the mesopterygoid fossa 

 continued forward between the posterior molars [Rhipidomys 

 Thomasomys, &c.), the doubtful species are more numerous 

 and I find the whole group needs revision, owing to the 



