338 ME. O. THOMAS ON MAMMALS FBOM CENTRAL PERU. [Apr. 18, 



each other that tlieir faces form a V in horizontal section, and 

 owing to this and to an apparently greater thickness of the enamel 

 at their outer as compared to their inner margins, their cutting- 

 edges also form a vertical reversed A, the outer corners projecting 

 downwards as sharp divergent points, of obvious use in seizing 

 fish. Molars somewhat of the Hahrothrix type, but simpler, with 

 high crowns, and opposite, not alternating, cusps, of Avhich there 

 are six in the first and four in the second molar both above and 

 below, although the two most anterior below tend to fuse into one 

 another. ]\P above and below subtriaugular, of average propor- 

 tional size. 



Type : /. stolzmanni, described below. " Hahrothrix " hydrobates, 

 Winge, should also be included in the genus. 



This remarkable new genus is one of very great and special 

 interest, on account of the fact that its members are modified, not 

 merely for an aquatic life, as are many other rodents, but actually 

 for a predacious piscivorous one, almost unique within the 

 order. This fact is fortunately proved without question by the 

 discovery, in the stomach of the Peruvian specimen now before 

 me, of both scales and fish-bones, which have been identified by 

 my colleague, Mr. Boulenger, as those of Tetragono^nerus alosa, 

 Giinth., a fish with an average length of about six inches. 



That the habits of Ichtlnjomys were in some \iay very peculiar 

 might have been inferred from the striking modification of the upper 

 incisors, of which the sharp separated points would apparently be 

 almost useless for the gnawing phytophagous Hfe of most rodents, 

 whether aquatic or not, while the extreme degree of specialization 

 in the swimming characters would result iu an activity and speed 

 under water fully capable of rivalling that of fishes or batrachians, 

 and very different from that of ordinary water-rats. Fiber, however, 

 and Hydroniys, of Myomorph rodents, are also both about equally 

 endowed with swimming-powers, and although both are primarily 

 plant-eaters, yet the former certainly occasionally captures and eats 

 slow-swimming fishes ', and perhaps the latter also will prove to do 

 so, especially as it has a tendency towards the peculiar double- 

 pointed character of the upper incisors present in Ichthyomys, and 

 also a very decided resemblance to it in the general shape and 

 structure of the skull. At the same time neither of these forms 

 has the peculiar physiognomy of Ichthyomys, whose general ex- 

 pression bears a considerable resemblance to that of such exclu- 

 sively fish-eating mammals as Lutra, Myogale, and Potamogale. 



On the whole the balance of evidence appears to be in favour of 

 Ichthi/omys not being a plant-eater at all, but of its living entirely 

 on fishes,' batrachians, crustaceans, or other water animals. 



Dr. Wiuf^e, in the description of his Hahrothrix hydrobates, has 

 entered into detailed comparisons of it with many water-mammals, 

 but owing to his specimen having been a skin only, he was unable 

 to make any investigation as to the character and contents of the 

 intestines. But, nevertheless, his placing /. hydrobates in the 

 ' See Merriam. Tr. L. Soc. New York, ii. p. 187 (1884). 



