the Genus ^Icsomys. 591 



I now owe to the kindness of Dr. E. A. Goeldi of Para the 

 two spiny rats from Marajo mentioned incidentally in his 

 paper of 1901 on Mesomys ecaudatiis * ; and at first I supposed 

 tliem to represent a new genus, but soou found that they 

 corresponded so closely in all their characters wit!i the original 

 descri|)tion of Mesomys ecaudatus (except for the presence of 

 a lon^ tail), that they were certainly identical generically, 

 and probably specifically, with Natterer's animal. 



Giinther^s ""^ Echimys ferrugineus" \ is also assignable to 

 the same genus, and is closely allied to M. ecaudatus. 



The following is a short description of the genus, with 

 measurements of one of the Marajo specimens : — 



External form as in Proechhnys, but the ears short and 

 the feet short and broad, as in Loncheres, the fifth hind toe 

 reaching to the end of the first phalanx of the fourth. 

 Spinous system at a maximum. Tail long, well haired. 

 Cheek-teeth rounded, as in Proeckimys, each upper one with 

 five or six transverse lamina?, the lower premolar with five 

 and the lower molars with four oblique laminae. General 

 form of skull as in Loncheres^ the muzzle being similarly 

 short, narrow, and parallel-sided ; palatal foramina small ; 

 bullae flattened. 



Exactly as Wagner stated, Mesomys may be said to have 

 the skull, ears, and feet of Loncheres, with the teeth of Fro- 

 ec/iimys, to wliicli latter it is really no doubt most closely 

 allied. From the structure of its feet it /^s,"^ esumably an 

 arboreal form, like Loncheres, instead of bt^^^t,. terrestrial, 

 like Proechimys. It is true that there are lather more 

 laminge in the upper teeth than is usual in Proechimys, but 

 there is a certain amount of variation in the number of the 

 laminae, even in the same species \, so that this difference 

 does not nullify the strong general resemblance that the teeth 

 of Mesomys have to those of Proechimys. 



The following are the dimensions of a Marajo specimen 

 (an adult male, preserved in spirit) : — 



Head and body 163 mm.; tail 186; hind foot (s. u.) 30; 

 ear 11. 



Skull : greatest length 41 ; basilar length 33 ; zygomatic 

 breadth 22"5 ; nasals 12*4x5; interorbitul breadth 11; 

 mastoid breadth 20; palate length 15'3 ; diastema 9-4; 

 palatal foramina 3'8; length of upper tooth-series 6*7. 



• Bol. Mas. Para, iii. p. 170 (1901). 



t P. Z. S. 1870, p. 750, pi. Ixxiv. (animal). 



t Cf. r. racillator, TI10.S. Ann. & Mas?. Nat. Ili.st. (7) .\i. p. 490 (1903). 



