On a Third Genus 0/ (he Iclitliyomys Group. 121 



posscsse3 the typical serie.^. That 9|)ecies has been only 

 recently described by Dr. Major, who has always taken a 

 very special interest in the present group of voles, so that I 

 have had fi;reat pleasure in naming after him this peculiar 

 little species, discovered by his kinsman Mr. Robert. 



The presence ot" a pectoral pair of niammte is a very 

 unusual character in I'lti/int/Sj which is usually said to have 

 only the two inguinal j)airs. The tendency to have extra 

 molar enamel spaces connecting with each other is also an 

 interesting point about this species, even though variable to a 

 certain extent. 



LVII. — .1 Third Genus 0/ the Ichthyoinys Group. 

 By Olufii:ld Thomas, 



In a small collection of Costa Rican animals just received 

 IVom Mr. C. F. Underwood there occur two specimens of a 

 remarkable new Rodent, the subject of the following 

 description : — 



RuEOMYS, gen. nov. 



General external characters as in Ichthijomys. ]\Iuzzle 

 below nostrils complete, not cleft, and entirely hairy, without 

 trace of a naked median groove. Ears present, but very 

 small, buried in the tur. Feet apparently rather more broadly 

 webbed, the webs as well as the toes fringed with bristles. 



►Skull smoothly rounded, little ridged, less modified in 

 general outline than in either Ichthyomys or Anotomys ; the 

 profile normal and the nasals not bent upwards. Nasal 

 opening high and narrow. Anterior zygoma-root as in 

 Ichthyoutys ; the anteorbital toraujen even more widt-iy open. 

 Pterygoids in normal j)ropoition to each other — in ichthyoniys 

 the entopferygoid is unusually developed and tiie ectoptery- 

 goid almost obsolete. 



Incisors quite normal, narrow, slightly but evenly rounded 

 in front, not unlike those of Akodon or Perornyscus, therefore 

 strikingly different to the peculiar incisors of Ichthyomy.s, 

 fully described in l<sy;3 *. Lower incisors narrow, slender, and 

 j)ointed, though less so than in Anutomys. Molars as in 

 Ichthyomys. 



Type. Rheomys Undericoodi, described below. " Ichthyo- 

 mys " trichotiSf Thos., is also a member of Ixheomys. 



This genus, the third known of the group, is to the full as 

 highly specialized for an aquatic lite as Ichlhyumys and 



• r. Z. S. 18!)3, p. 3.'37. Anotonv/s was described Ami. ..^ Maj,'. Nat- 

 Hist. (7) xvii. p. bO, Jiuiuarv ISXMJ. 



