ANCESTRY. The medium in which fishes live and the hard and almost indestructible nature of some portions of their skeleton, as their teeth, spines. and scales, would lead us to anticipate their frequent occurrence in the sedimentary rocks: but inasmuch as the soft parts of the animal are liable to speedy decomposition, the remains of fish must often exist in a fragmentary and scattered condition. Thus the teeth in the shark, the spine defense in the stingray, and the scales in the bony pike would survive the total destruction of the cartilaginous skeleton as well as the soft portions. Many quite complete casts of skeletons, however, have been obtained, so that not a little is known of the past history of the group. The earliest fishes occurred in the Upper Silurian. of all of the main groups, excepting the higher teleosts, have been found from this period. Among the ehismobranchs the earlier forms were quite distinct from any now living, with the possible exception of the Port Jackson shark (q.v.). These forms flourished
to the Triassic period, and in the case of the ces tracionts to the Eocene. The recent elasmo branchs appeared in the late Triassic or early Jurassic, and were more abundant in the past than at present. The Dipnoi flourished in the Triassic. The ganoids were a dominant group up to Miocene times, but at. present exist in mere remnants. The dominant fishes of to-day, name ly, the higher teloosts, first appeared in num bers during the lurassic and Cretaceous periods. These at the present clay exhibit the greatest di versity in type. In times past the other groups presented this great variety of form, and it is mainly those species that retained the more gen eralized characters that survived and are present with us now. See EXTINCTION OF SPECIES.