1S93.] ME. O. THOMAS Olf MAMMALS FEOM CENTRAL PERU. 339 



genus or subgenus Habrothrix appears to me quite unaccountable, 

 especially as he is one of the authors who consider that the 

 majority of the different groups of what used to be called 

 " Hesperomiis " should rank as distinct genera. A short diagnosis 

 of Hahrotlirix, based on the typical species, HJonrjipilis, is given in 

 P. Z, S. ISS-l, p. 450, and it will be seen that Ic7Uhi/onujs stolzmanni 

 and /. hi/drobates agree with that diagnosis in scarcely a single 

 character of importance, even though it was not drawn up with 

 the idea of any antithesis to such a specialized swimming form as 

 the present. 



The xevy noticeable resemblance in the structure of the anterior 

 zygoma-root \\hich IcJtihi/omi/s bears to JJ>/drom>/s is a point well 

 worthy of remark, for \\hile this resemblance practically amounts 

 to identity, yet there can be no suspicion that the t^^■o have a 

 common origin, or can be other than a very remarkable case 

 of parallelism. This case is the more remarkable as the structure 

 of this region has been used by all the best authorities as a cha- 

 racter of primary importance in dividing the Myomorph rodents 

 into smaller groups, so that it will not be readily looked upon as 

 one of little stability. Nevertheless, in the present instance we 

 have two Murines, alike in their mode of life but derived from 

 quite different ancestors, developing independently exactly similar 

 infraorbital foramina \ The presumed ancestor of each of 

 the two highly specialized forms under consideration, Xeromys of 

 Hydromi/s, and Hahrothrix (or seme ally) of IcJitliyomys, both 

 have typically murine infraorbital foramina. 



IcHTHYOMTS STOLZM.iJs'xi, sp. n. (Plate XXVIII.) 

 Size and general proportions much as in the common Black Eat 

 {Mus rattus). Whiskers long, strong, and numerous, silvery wliite 

 with the exception of a few of the upper ones, ^hich are bi-own. 

 Ears very small and narrow, when laid forA^ard they do not reach 

 halfway towards the eye. Hands with the fingers quite free and 

 unwebbed ; pollex with a sort of elongated nail, not long enough 

 to be called a claw ; other digits M'ith sharp curved claws ; third 

 and fourth fingers subequal, second reaching to the middle of the 

 second phalanx of the third, fifth to the base of the same phalanx 

 of the fourth ; palm naked, Avith the usual five pads. Hind feet 

 broad, fan-shaped ; unwebbed ])art of toes broadly ciliated margin- 

 ally ; soles naked, witli five large low rounded pads, the usual 

 small postero-external pad absent. Tail about as long as the head 

 and body, thick, cylindrical, its terminal half below with stiff' 

 elongate bristles. Palate-ridges 3 — 3. Mammaj doublful, owing 

 to part of the alKlominal wall having been cut away, but there is 

 one pair just behind the axilla;, and another near the vulva ; 



' Dr. Wingo (/. c. p. 22) ingpniouHly suggests, as tlie reason of the enlargc- 

 uicnt in the lower part of the foramen, tlie iiicroascd size of the nerve which 

 supplies the prominent wliisker-bristles. Although jig doubt true so far aa it 

 goes, this explanation does not aj)pear to me (juite to cover the case, especiully 

 us Fiher, similar in habits and with nearly equal whisker development, has a 

 highly typical Murine f(jranien. 



