{12}_Fresh-water fishes._--The great rivers of Tropical America abound in fish of many strange forms and peculiar types. Three families, and three sub-family groups are peculiar, while the number of peculiar genera is about 120. The peculiar families are Polycentridæ, with two genera; Gymnotidæ, a family which includes the electric eels, (5 genera); and Trygonidæ, the rays, which are everywhere marine except in the great rivers of South America, where many species are found, belonging to two genera. Of the extensive family Siluridæ, three sub-families Siluridæ anomalopteræ, S. olisthopteræ, and S. branchiolæ, are confined to this region. The larger and more important of the peculiar genera are the following: _Percilia_, inhabiting Chilian and _Percichthys_ South Temperate rivers, belong to the Perch family (Percidæ); _Acharnes_, found only in Guiana, belongs to the Nandidæ, a family of wide range in the tropics; the Chromidæ, a family of exclusively fresh-water fishes found in the tropics of the Ethiopian, Oriental and Neotropical regions, are here represented by 15 genera, the more important being _Acara_ (17 sp.), _Heros_ (26 sp.), _Crenicichla_ (9 sp.), _Satanoperca_ (7 sp.). Many of these fishes are beautifully marked and coloured. The Siluridæ proteropteræ are represented by 14 genera, of which _Pimelodus_ (42 sp.), and _Platystoma_ (11 sp.), are the most important; the Siluridæ stenobranchiæ by 11 genera, the chief being _Doras_ (13 sp.), _Auchenipterus_ (9 sp.), and _Oxydoras_ (7 sp.). The Siluridæ proteropodes are represented by 16 genera, many of them being among the most singular of fresh-water fishes, clothed in coats of mail, and armed with hooks and serrated spines. The following are the most important,--_Chætostomus_ (25 sp.), _Loricaria_ (17 sp.), _Plecostonus_ (15 sp.) and _Callichthys_ (11 sp.). The Characinidæ are divided between Tropical America and Tropical Africa, the former possessing about 40 genera and 200 species. The Haplochitonidæ are confined to South America and Australia; the American genus being _Haplochiton_. The Cyprinodontidæ are represented by 18 genera, the most important being, _Pæcilia_ (16 sp.), _Girardinus_ (10 sp.), and _Gambusia_ (8 sp.) The Osteoglossidæ, found in Australian and African rivers, are represented in South America by the peculiar _Arapaima_, the "pirarucu" of the {13}Amazon. The ancient Sirenoidei, also found in Australia and Africa, have the _Lepidosiren_ as their American representative. Lastly, _Ellipisurus_ is a genus of rays peculiar to the fresh waters of South America. We may expect these numbers to be largely increased and many new genera to be added, when the extensive collections made by Agassiz in Brazil are described.