1889.] MR. O. THOMAS ON A NEW GENUS OF MURID^. 249 



lobe of rn^ is slightly folded inwards at its centre. Lower incisors 

 very Ions:, their front surface white. Lower molars (fig. 4) as in 

 Hydromi/s, but the walls of the large anterior lobe of m7' are notched, 

 so as to give a rather more cuspidate character to the tooth. 



Dimensioi!S of the type, an adult female in spirit : — 



Head and body 111 milhm. ; tail 85 ; hind foot 23-2 ; ear 10 X 

 10'.5 ; head 33 ; forearm and hand 29 ; heel to front of last foot-pad 

 10-8 ; length of last foot-pad 2-G. 



Skull. — Basal length 26 ; greatest breadth 15; nasals, length 8*9, 

 breadth 3"3 ; interorbital breadth 5'0 ; interparietal, length 3*1, 

 breadth 92 ; infraorbital foramen, length of outer wall 2"^, distance 

 from outer corner of one foramen to that of the other 7"6 ; palate, 

 length l(j, breadth outside m.^ 5".5, inside m.' 2-1 ; diastema 9 ; 

 length of palatine foramina 3*9 ; length of ^^ 2"8, of nx/" 1'5 ; of 

 the two together in situ 4'1. 



Bub. Port Mackay, Queensland (Godeffroy Museum). 



The above given being the combination of characters presented by 

 the new form, we may turn to the interesting questions as to the phylo- 

 geny of Hydromys naturally raised by its discovery. Had the origin 

 of Hydromys been formulated apart from Xeromys, it would most 

 assuredly have been somewhat as follows : — The ancestor oi Hydromys 

 would have been said to have been an ordinary Murine with three 

 molars, which took to an aquatic life as Mas fuscipes, Microtus 

 amphibius, and others have done, and that then, afterwards, as the 

 external characters became modified for swimming, and as some 

 water-loving substance was more and more exclusively used as food, 

 the teeth became modified in the remarkable manner characteristic 

 of the genus. This natural speculation, apparently quite sound in 

 itself, is abruptly overthrown by the discovery of Xeromys ; for that 

 animal, without having developed the aquatic habits and characters 

 of Hydromys, has already attained to the same specialized dental 

 peculiarities. That Xeromys is the almost unmodified descendant 

 of one of the more recent direct ancestors of Hydromys is almost 

 unquestionable, as it does not possess a single peculiar character of its 

 own, every one of its points being present either in its relatives the true 

 Rats and Mice, or in what we may fairly call its offspring, Hydromys. 



The true course of the evolution of Hydromys appears therefore to 

 have been this. There would have been living in Australia, perhaps 

 comparatively recently, one or more species of a terrestrial genus 

 possessing a Murine exterior and skull, and Hydromyiue dentition, 

 palate-ridges, and mammae {i. e. Xeromys as now defined). Some 

 members of this genus taking to an aquatic life, such of their 

 characters as had any direct relation to the power of swimming 

 would have become modified, these being size, form of head, and 

 therefore of skull, structure of muzzle (for cleaving the water and 

 keeping it out of the mouth), great whisker development, closeness 

 and glossiness of fur, extra folds on ear-conch, webbing of toes, 

 suppression of sole-pads, and hairiness and increase in size and 

 strength of tail. On the other hand, the number and structure of 

 the teeth, and even such slight and presumably easily modified 



