CHAP. XX.] FISHES. 465 



latter family has a single species iu the Indian seas. Among the 

 curiosities of distribution are, — the extensive genus Diagramma, 

 confined to the Pacific with the exception of one species in the 

 Mediterranean ; the single species constituting the family Lopho- 

 tidee, found only in the Mediterranean and Japan ; the small 

 family of Notacanthi, confined to Greenland, the Mediter- 

 ranean, and West Australia; and the four families, Sternop- 

 tychidse, Stomiatidse, Alepocephalidse, and Halosauridae, which 

 are believed to inhabit exclusively the depths of the ocean, and 

 are therefore very rarely obtained. 



Fresh-water Fish. — There are 36 families of fishes which 

 inhabit fresh water exclusively, and 5 others, which are both 

 marine and fresh-water. These present many interesting pecu- 

 liarities of distribution. The Neotropical region is the richest 

 in families, and probably also in genera and species. No less 

 than 22 families inhabit it, and of these 6 are altogether peculiar. 

 The Ethiopian and Nearctic regions each have 18 families, the 

 former with 3, and the latter with 5 peculiar. Several isolated 

 forms, requiring to be placed in distinct families, inhabit the 

 great American lakes ; and, no doubt, when the African lakes 

 are equally well known, they will be found also to possess many 

 peculiar forms. The Oriental region comes next, with 17 families, 

 of which 3 are peculiar. The Palaearctic has 12, and the Aus- 

 tralian 11 families, each with only 1 altogether peculiar to it. 



If we take those regions which are sometimes supposed to be 

 so nearly related that they should be combined, we shaU find the 

 fresh-water fishes in most cases markedly distinct. The Nearctic 

 and Palaearctic regions, for example, together contain 20 families, 

 but only 11 of these occur in both, and only 5 are exclusive 

 inhabitants of these two regions. This shows an amount of 

 diversity that would not, perhaps, be exhibited by any other 

 class of animals. The Ethiopian and Oriental regions together 

 possess 24 families, only 11 of which are found in both, and 

 only 1 exclusively characteristic of the two. The Australian 

 and Neotropical regions possess together 27 families, of which 7 

 are found in both, and 3 are exclusively characteristic of the 

 two. This last fact is very interesting : the marine family of 



