1889.] MR. 0. THOMAS ON A NEW GENUS OF MURID#. 249 
lobe of m1 is slightly folded inwards at its centre. Lower incisors 
very long, their front surface white. Lower molars (fig. 4) as in 
Hydromys, but the walls of the large anterior lobe of m.1 are notched, 
so as to give a rather more cuspidate character to the tooth. 
Dimensions of the type, an adult female in spirit :— 
Head and body 111 millim.; tail 85 ; hind foot 23°2; ear 10X 
10°5 ; head 33; forearm and hand 29; heel to front of last foot-pad 
10°8 ; leagth of last foot-pad 2°6. 
Skull.—Basal length 26; greatest breadth 15; nasals, length 8:9, 
breadth 3°3; interorbital breadth 5:0; interparietal, length 3:1, 
breadth 9-2; infraorbital foramen, length of outer wall 2°8, distance 
from outer corner of one foramen to that of the other 7:6; palate, 
length 16, breadth outside m.* 5:5, inside m.? 271; diastema 9; 
length of palatine foramina 3:9; length of m1 2°8, of m? 1:5; of 
the two together in situ 4°1. 
Hab. 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 of Hydromys 
would have been said to have been an_ordinary Murine with three 
molars, which took to an aquatic life as Mus 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 ydromys 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, Zydromys. 
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 Hydromyine dentition, 
palate-ridges, and mamme (i.e. Xeromzys 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 
