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H YD ROM YS-H YDROPHILID^ 



and the structure of their legs, formed for aquatic 

 life, they are more properly Hydrocorisce, from which, 

 however, they differ in residing upon the surface, and 

 not in the water. For this purpose it is necessary 

 that they should be defended against the action of the 

 waves in the most complete manner, and which we 

 find effected in a two-fold manner. In the typical 

 genus, Hydronu'tra, the legs are simply formed for 

 walking, but the insect is considerably elevated by 

 them above the surface, and is enabled easily in this 

 manner to make its way through grass and the stems 

 of plants growing at the edges of ponds, &c., which 

 is its proper locality ; whereas the other genera 

 (Gerris, Velio) rest their bodies upon the surface of 

 the water, but they are defended from its action by a 

 coating of silky hair, over which the water glides 

 without affecting the body of the insect these, more- 

 over, are provided with long rowing legs, which enable 

 them to slum rapidly along the surface of the water. 

 The genus Hydromctra is extremely slender, and the 

 head is produced into a long, narrow muzzle. The 

 type is the Hydromctra stagnorum, which is an opake 

 black colour, and more than half an inch in length. 

 For the characters, &c. of GERRIS, see the article 

 upon that genus. 



HYDROMYS. A genus of rodent mammalia, 

 hitherto found only in the southern part of Australia, 

 and we believe only in Van Diemen's Land. They 

 are remarkable among the mammalia of that country 

 in not being marsupial. As they have been hitherto 

 seen on the south coast of Van Dieman's Land only, 

 it is possible that they may be importations which 

 have been brought there by visitors from some other 

 part of the world, whose appearance has been forgot- 

 ten by a race so totally without historic recollections, 

 and so prone to extirpate any strangers that come 

 among them, as the original inhabitants of that island 

 unquestionably are. Though these animals are called 

 liydromys, or water mice, they do not exactly agree 

 w ith our water mice, nor with those of any other part 

 of the world ; they are characterised by having five 

 toes on each of the feet, all free on the fore feet, and 

 webbed on the hind ones ; all the toes are furnished 

 with claws, which are small and pointed, except that 

 on the thumb of the fore foot, which is flattened, and 

 the thumb itself is but little produced ; their teeth are 

 also peculiar ; they have only twelve altogether, 

 namely two incisors, and four cheek teeth in each 

 jaw ; the upper incisors are united and flat in their 

 anterior surfaces, and the same surface of the lower 

 ones is rounded ; the first grinder -in the upper jaw is 

 always longer than the second, and the crown of the 

 first is composed of three, and that of the second of 

 two depressions, surrounded by elevated portions of 

 enamel ; the two in the under jaw are formed in 

 nearly a similar manner, and the first one is double 

 the size of the second. This system of teeth is among 

 the most simple which is met with among mammalia, 

 excepting in those species which are reckoned tooth- 

 less ; the ears are very short, and rounded ; the tail 

 is round, and covered with .short hair ; the body is 

 covered with two sorts of hair, one woolly and very 

 soft to the feel, and another much longer and thicker 

 in the single hair, and shining and silky in appear- 

 ance. Their covering thus makes a slight approxi- 

 mation toward that of the South Sea seals, which are 

 so abundant in the offing of the places where the pre- 

 sent genera are found on the shores. Only two spe- 

 cies have been described, and it is not very well 



ascertained whether these ought to stand as distinct 

 species, or only as varieties, though the former is per- 

 haps the more correct view of them. These species 

 are distinguished as Leucogaster, or white belly and 

 Crysogaster, or yellow belly. 



The first has the body about a foot in length, and 

 the tail about eleven inches. All the upper part is 

 brown, and the under part, which is also the colour of 

 the distal half of the tail. In this one, the hind feet 

 have webs for only half the length of the toes, which 

 would indicate an animal of not absolutely aquatic 

 habits. The fur all over it is very soft and delicate 

 to the touch. 



The other species is nearly of the same size, and 

 differs from it chiefly in the colour of the belly, and 

 that of the tail, which has only the tip white. The 

 fur upon this last mentioned one is also considerably 

 finer than that upon the other ; but enough is not 

 known of them for warranting us to say positively 

 whether they are, or are not, separate species. The 

 places where they have been seen, have been chiefly, 

 if not exclusively, the small islands which lie in the 

 entrances to the river Derwent, and render those en- 

 trances so dangerous to shipping. It is supposed 

 that they are not numerous ; and we have mentioned 

 that they may be foreigners ; and the chief point of 

 interest about them is, that they stand almost alone 

 as placental mammalia, in a country where all the 

 rest are marsupial, with the exception of the dog ; 

 and we have endeavoured to show in the article Dog 

 that the dingo, or wild dog of Australia, is an impor- 

 tation by the Malays. The curious fact of the animals 

 of Australia, in both of its islands, being almost as ex- 

 clusively marsupial as those of the rest of the world 

 are placental ; and the connexion which this has with 

 the singular physical nature of the country are, how- 

 ever, points which we cannot decide without much 

 more information than we are in possession of at 

 present. 



HYDROPELTIDEvE. A natural order of plants, 

 comprising a single genus, of which there is only one 

 species described ; but of the properties there is 

 nothing certain known. Hydropeltis is an aquatic, 

 and intimately connected with Caltha ; and a kindred 

 genus, called Cabomba, bears a great similitude to the 

 Batrachia, even in its heteromorphoses foliage ; the 

 floating leaves being lobed, while the immersed ones 

 are very much divided. De Candolle mentions their 

 affinity to the Nymphceace<, with which they are 

 associated bv Bartling. 



HYDROPHILID.E (Leaeh). A family of cole- 

 opterous insects, belonging to the section Pentama-a, 

 and division Rypopliaga of Stephens (Philhydrkla 

 Mac Leay), comprising such species of water-beetles 

 as have the antennae short and clubbed at the ex- 

 tremity, and the palpi long and slender ; the mandi- 

 bles are bidentate at the tips, the body is oval and 

 convex, and the hind legs often -ciliated. Like the 

 Dyticidte, which also inhabit the. water, these insects 

 exhibit but little variation in their colours, which are 

 ordinarily obscure. The consistence of their bodies 

 is firm, and they are often armed with a strong acute 

 spine, which extends between their hind legs. They 

 are furnished with wings, whereby they are able to 

 transport themselves from one piece of water to 

 another, when they are so inclined. Their motions 

 in the water are .much less active than those of the 

 Dyticidte, the legs -not being, moved simultaneously. 

 This want of agility is dependant in a great measure 



