416 Dr. J. E. Gray on 7?a^f. 



LII. — Notes on the Rats ; with the Description of some new 

 Species from Panama and the Aru Islands. By Dr. J. E. 

 Gray, F.R.S. &c. 



The species of Rats are exceedingly difficult to distinguish ; 

 and from many species having the habit of going on board 

 ships, and of being carried about in goods, the same species 

 becomes spread over different parts of the world, rendering it 

 very unsafe to distinguish them by their geographical distribu- 

 tion. The skulls afford one of the best characters to separate 

 the species into groups ; and therefore I have arranged the 

 various skulls in the British Museum thus, and described three 

 species which I did not find recorded in the catalogues in such 

 a manner tliat I could recognize them. 



The British Museum has lately procured from Paris a rat 

 from Panama, which I do not find described, and am therefore 

 inclined to think is new. It is well distinguished from all 

 other rats by the form of the skull, and especially of the cavity 

 on the side of the nose in front of the orbit. 



Neomts. 



Head short, rather conical. Whiskers long, smooth, black. 

 Eyes moderate. Ears rounded, naked. Fur very soft, close, 

 with very slender, rather stiffer, black hairs. Feet covered 

 with very short close hair. Toes quite free, four in front, five 

 behind, with short curved claws, which are partly covered with 

 white hairs at the base. Tail elongate, naked, slender, covered 

 with close, regular, thin, square scales placed in rings ; black, 

 the terminal third i)eing white. 



Skull broad, depressed, nearly twice as long as wide, flat 

 above, with a thin edge above the eyebrow, which is continued 

 on and expanded out, forming a Avide sharp keel on the sides 

 of the temples. The aperture in front of the orbit narrow, 

 linear, erect, with a sharp, nearly straight, slightly arched 

 hinder edge, and a large concavity in front, which is not so 

 long as high and has a slanting ridge in front, so that it is 

 much naiTower at the lower part than the hinder one, and 

 has an arched lower edge ; there is a tubercle in the middle of 

 the front edge. Teeth |^, as in Mus. 



This genus is at once known from the true genus Mus by 

 the softness and slenderness of the longer hair and the general 

 softness of the fur. The skull is at once distinguished from 

 that of the genus Mus by its very flat broad crown, which has 

 a rather broad expanded edge over the eyes and the upper part 

 of the temporal muscles, and in the form of the aperture in front 



