218 Mr, 0. Thomas on 



an instance of the theory of " repulsion ■" (described P. Z. S. 

 1894, p. 144), by which when two allied species live together 

 in the same place, and can gain no advantage by mutual 

 resemblance (" mimicry '^), they often intensify their colour 

 differences to the greatest possible extent, in order probably 

 that their members shall be enabled to distinguish comrades 

 from rivals as readily as possible. 



XX. — Descriptions of Four new South- American Mammals. 

 By Oldfield Thomas. 



Oxymycterus lanosus, sp. n. 



General appearance exceedingly like that of Akodon xan- 

 thorhinus, Waterh. Fur very soft, thick, and woolly, the 

 wool hairs about 9 millim. long on the back, and the longer 

 straight hairs forming a thick fringe 5 or 6 millim. beyond 

 the wool hairs. General colour deep yellowish olive all over 

 above, the sides brighter yellow along the junction with the 

 belly. Under surface slaty grey, the tips of the hairs buffy 

 white. Ears small, not projecting above the fur, well haired. 

 Upper surface of hands and feet shining white ; pollex with 

 a blunt claw, hardly long enough to be called a true claw, 

 but longer and more compressed than a " nail." Fifth hind 

 toe reaching to the middle of the basal phalanx of the fourth. 

 Tail about as long as the body without the head, well haired, 

 blackish brown above, yellowish white below and on the 

 sides. 



Skull with a narrow slender muzzle and a very large 

 rounded brain-case. Nasals narrow, pointed, concave above 

 when viewed in profile. Interorbital region broad, smoothly 

 rounded, convex above, without ridges. Brain-case broad, 

 flattened, rounded. Interparietal small. Anterior zygoma- 

 root very narrow, slanted forwards. Anterior palatine fora- 

 mina reaching back one third of the length of m^ j hinder 

 edge of palate opposite back of in. 



Incisors narrow, slender, very pale yellow above and 

 white below. Molars as usual. 



Dimensions of the type, in skin, male : — 



Head and body (c.) 80 millim. ; tail 51 ; hind foot (moist- 

 ened) 20-6. 



Skull : basilar length 1 8*4, greatest breadth (across brain- 

 case) 12*4; nasals 9-3 x 2*8 ; interorbital breadth 4-8; 

 palate length from henselion 9*9 ; diastema 6*1 ; palatal 

 foramina 5"3 x 2 ; length of upper molar series 4. 



IJah. Monteith Bay, Straits of Magellan. 



Type. B.M. 80.7.28.11. Collected and presented by Dr. K. 

 W. Coppinger, of H.M.S. ' Alert.' 



