the outwardly and horizontally directed pectoral and pelvic fins, modified to form an efficient adhesive surface for attaching the Fish to the stones and rocks of the river bottom[121] (Fig. 355). In the males of Elasmobranchs, except in the Palaeozoic Shark _Cladoselache_, and of Holocephali, the hinder portions of the pelvic fins are modified to form copulatory organs, the claspers, mixipterygia, or pterygopodia. Lastly, it may be mentioned that the spines, often long, pointed, and sometimes serrated, with which the paired and median fins of many Fishes are provided, furnish formidable offensive or defensive organs, especially when they are associated with poison glands, and also that in by no means an inconsiderable number of Teleosts the spines may form part of a stridulating vocal mechanism. In different Fishes the pectoral and pelvic fins and the {163}median fins may, individually, all be absent through atrophy. The pectoral fins are rarely absent: nevertheless, in certain species of Syngnathidae, and in most Muraenidae, for example, these fins are entirely wanting. The pelvic fins are much less constant and are often absent in entire families, as in the Pipe-Fishes (Syngnathidae), the "Electric Eels" (Gymnotidae), and the true Eels (Anguillidae), and in the Globe-Fishes and Porcupine-Fishes (_Tetrodon_, _Diodon_), as well as in certain genera of families where they are usually present, as in some of the Blennies (Blenniidae) and in the Ophidiidae. Even when present the pelvic fins are often reduced to mere vestiges in the shape of filaments, as in some of the Gadoids (Gadidae) and Ribbon-Fishes, or are represented only by a pair of defensive spines, as in some Sticklebacks (_Gastrosteus_), or even by a single spine (Balistidae). Complete suppression of the pelvic fins, or their reduction to vestigial remnants, seems to be of frequent occurrence in Fishes which live in the mud, or are able to pass a longer or shorter time in soil periodically dried during the hot season, as in some Cyprinodontidae, and in species of such tropical Teleostean families as the Ophiocephalidae, Galaxiidae, and Siluridae. Suppression of the dorsal fin is apparent in the Gymnotidae, and of the anal fin in the Ribbon-Fishes. In some of the latter family, as in the rare British visitor the Oar-Fish (_Regalecus banksii_), and in the Sea-Horse (_Hippocampus_), where the tail becomes a prehensile organ for coiling round seaweeds when the animal is not swimming, the otherwise remarkably constant caudal fin is absent. An initial stage in the degeneration of median fins is to be seen in many of the Salmonidae and Siluridae, in which a posterior division of the dorsal fin becomes reduced in size, loses its fin rays, acquires much fat in its substance, and becomes an "adipose fin."