1893.] ME. O. THOMAS OS^ MAMMALS FROM CENTRAL PEETJ. 339 



genus or subgenus Hahrothrix appears to me quite unaccountable, 

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

 majority of the different gi-oups of what used to be called 

 '^ Hesperomys" should rank as distinct genera. A short diagnosis 

 o£ Habrothrix, based on the typical species, H.lonr/i^nlis, is given in 

 P. Z. S. 1884, p. 450, and it will be seen that Ichthyomys stolzmanni 

 and /. hydrobates 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 very noticeable resemblance iti the structure of the anterior 

 zygoma-root which Iclithyomys bears to Hydromys is a point well 

 worthy of remark, for while this resemblance practically amounts 

 to identity, yet there can be no suspicion that the two 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 

 Hydromys, and Hah roth riv (or some ally) of Ichthyomys, both 

 have typically murine infraorbital foramina. 



Ichthyomys stolzmaotsti, sp. n. (Plate XXVIII.) 

 Size and general proportions much as in the common Black Eat 

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

 with the exception of a few of the upper ones, which are brown. 

 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 with 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, with the usual five pads. Hind feet 

 broad, fan-shaped ; unwebbed part of toes broadly ciliated margin- 

 ally ; soles naked, with 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—8. Mammae doubtful, owing 

 to part of the abdominal waU having been cut away, but there is 

 one pair just beliind the axillae, and another near the vulva ; 



1 Dr. Winge {t. c. p. 22) ingeniously suggests, as the reason of the enlarge- 

 ment in the lower part of the foraiuen, the increased size of the nerve which 

 supplies the prominent whisker-bristles. Although no doubt true so far as it 

 goes, this explanation does not appear to me quite to cover the case, especially 

 as Fiber, similar in habits and with nearly equal whisker development, has a 

 highly typical Murine foramen. 



