FRINGE-FINNED GROUP. 517 



istic of the Secondary period, their remains being especially common in the British 

 Lias. In both of the two principal families the tail is of the heterocercal type. 

 In one family, as typified by the genus Palceoniscus, the body is elongated fusiform, 

 and the teeth are slender and conical or straight. On the other hand, Platysomus 

 represents a second family (Platysomatidce), in which the body is rhomboidal, 

 and the teeth — in the upper jaw mainly confined to the pterygoid bones — obtuse. 

 In both groups the scales are of the ganoid type. 



The Fringe-Finned Ganoids, — Order Crossopterygii. 



The whole of the members of the subclass under consideration described in 

 the foregoing pages constitute one great order (Actinopterygii), characterised, as 

 mentioned on p. 334, by the fan-like structure of the paired fins, and frequently 

 also of the caudal fin ; the scales being generally of the cycloid or ctenoid type. 

 These fishes form, indeed, the dominant group at the present day ; whereas the group 

 now to be considered is represented only by two existing species — referable to as 

 many genera, and is mainly characteristic of the earlier epochs of the earth's 



THE BICHIR. 



history, being abundant even in the Devonian and Carboniferous epochs, since 



which time it has been steadily decreasing in numbers. These fringe-finned 



ganoids, as they may be called, have the paired fins lobate, with an internal 



longitudinal axis belonging to the true skeleton more or less fringed with dermal 



rays, the caudal fin being either of the diphycercal or heterocercal type. A pair 



of large jugular plates, bounded in some instances by a series of smaller lateral 



ones, and an anterior unpaired element, are developed in the branchiostegal 



membrane to fill up the space between the two branches of the lower jaw, and 



thus representing the branchiostegal rays of the first order. In all the scales are 



coated with ganoine, although they may be thin, overlapping, and rounded, or thick 



and quadrangular. The existing forms have the optic nerves simply crossing one 



another, a spiral valve in the intestine, and a duct to the air-bladder ; the presence 



of the latter being also shown in certain extinct types. Next to the sharks and 



rays, this group is one of the oldest, being well represented in the Devonian. 



The sole existing: survivors of this great group of fishes are the 

 Existing Species. . ° . P . 



bichir (Polypterus bichir) of the Nile, and other rivers of Tropical 



Africa, and the reed-fish (Calamoichthys calabaricus) from Old Calabar; these 



constituting the family Polypteridce, which has no fossil representatives, and 



probably forms a subordinal group by itself. In this family the notochord is more 



or less constricted and replaced by ossified vertebrae; the baseosts, or superior 



supporting elements, are rudimentary, or wanting, in the median fins ; whereas the 



