It must not be imagined that these three categories are more sharply defined than Freshwater and Marine Fishes. They gradually pass into each other, and there are numerous fishes about which uncertainty exists whether they should be placed in the Shore or Pelagic series, or in the Pelagic or Deep-sea series; nay, many facts favour the view that changes in the mode of life and distribution of fishes are still in progress. The change in habitat of numerous fishes is regulated by the distribution of their favourite food. At certain seasons the surface of the sea in the vicinity of land swarms with mollusks, larval Crustaceans, Medusæ, attracting shoals of fishes from the open ocean to the shores; and these are again pursued by fishes of larger size and predacious habits, so that all these fishes might be included, with equal propriety, in the littoral or pelagic series. However, species which are known to normally spawn in the open ocean must be always referred to the latter division. Chondropterygii, Acanthopterygii, Anacanths, Myxinoids, and Pharyngobranchii furnish the principal contingents to the Marine Fauna; whilst the majority of Malacopterygians, the Ganoids, and Cyclostomes are Freshwater Fishes. I.--DISTRIBUTION OF SHORE FISHES. The principal types of Shore-fishes are the following:-- CHONDROPTERYGII-- HOLOCEPHALA 4 species PLAGIOSTOMATA-- _Carchariidæ_ (part.) 12 „ _Scylliidæ_ 30 „ _Cestraciontidæ_ 4 „ _Spinacidæ_ (part.) 8 „ _Rhinidæ_ 1 „ _Pristiophoridæ_ 4 „ _Pristidæ_ 5 „ _Rhinobatidæ_ 14 „ _Torpedinidæ_ 15 „ _Rajidæ_ 34 „ _Trygonidæ_ 47 „