920 CLASS PISCES. 



girdle. In the Dipnoi, and especially in Ceratodus, the pectoral 

 (fig. 845) and pelvic fins are supported by a cartilaginous, median, 

 segmented axis, bearing jointed radii on the dorsal and ventral 

 borders — these radial cartilages being terminated by horny dermal 

 fin-rays ; and the dorsal radii (left side of figure) being more numer- 

 ous than the ventral. This type of fin, which also occurs in the 

 Ichthyotomous Elasmobranchei, is known as the archipterygium. 

 From this slightly unsymmetrical type of fin that of existing Sel- 

 achian Elasmobranchs (fig. 846) may be derived by the gradual sup- 

 pression of the ventral series of rays, and the development of the 

 dorsal, which has now become lateral. Basally the jointed radial car- 

 tilages articulate proximally with the pro-, meso-, 

 and metapterygium, which in their turn are at- 

 tached to the pectoral arch, and the latter of 

 which corresponds to the basal axial cartilage 

 of the fin of Ceratodus (fig. 845). In the 

 pelvic fin of the Selachians the mesopterygium 

 is absent, and the propterygium more or less 

 rudimentary. This type of fin is known as the 

 ichthyopterygium. The fins of Ganoids and 

 Teleosteans may be derived from the Selachian 

 type ; but the primary cartilaginous skeleton is 

 m of reduced, and a secondary one developed by the 

 the left pectoral limb of introduction of membrane bones. 



theMonkhsh(Sguatzna). . . =. 



p, Pro-; vis, Meso-; mt, Before leaving the structure of Fishes, men- 



Metepterygium. Much ^ mugt be made Qf ^ ^^ whkh ^ 



small, rounded, elliptical bodies, usually with one 

 convex and one concave side, lying in the tympanic sac, and com- 

 posed of both calcic carbonate and phosphate. These bodies have 

 been carefully studied by Dr Koken, and several genera identified 

 by their evidence in a fossil state. 



As regards their distribution in time, Fishes being the lowest 

 class of the Vertebrata, it would naturally be supposed that they 

 were the earliest representatives ; and this appears to have been the 

 case. The earliest known fishes in Britain belong to the Ganoid 

 group Placodermata, and occur in the Lower Ludlow group of the 

 Silurian ; while the Elasmobranchei were represented in the topmost 

 group of the same series. In the Devonian and Carboniferous 

 periods Fishes become abundant ; but all the forms from these 

 horizons, and up to the Cretaceous, belong to the Elasmobranchei, 

 Chimseroidei, Dipnoi, and Ganoidei — the specialised Teleostei not 

 making their appearance, so far as we know with certainty at present, 

 till the Cretaceous. The Ganoids of the suborder Amioidea ap- 

 proximate, however, so closely to the Teleostei, that it has been a 

 question whether some of the members of the Jurassic family Lepto- 



