6 



The more important genera of Placoderms are, Diniclitliys, 

 AspidicIitJiys, and Diplognathus, our largest Palseozoic fishes. 

 Others are, Acanthaspis, Acantholepis, Coccostens, Macropetalich- 

 thys, and Onychodus. Among the Elasmobranchs were, Clado- 

 dus^ Cienacanthus^ Machmracmithus^ Rhynchodus, and Ptyciodus, 

 the last two being regarded as Chimseroids. In the Chemung 

 epoch, the great Dipterian family was introduced with Dipterus^ 

 tleliodus^ and possibly Ceratodus. Species of the European 

 genera, Bothriolepis and Holopiychius, have likewise been found 

 in our Devonian deposits. 



With the close of the Devonian, came the almost total extinc- 

 tion of the great group of Placoderms, while the Elasmobranchs, 

 which had hitherto occupied a subordinate position, increase in 

 numbers and size, and appear to be represented by Sharks, 

 Rays, and Chima=;ras. Among the members of this group from 

 the Carboniferous, were numerous Cestracionts, species of 

 Cochliodus of large size, with others of the genera Deltodus^ 

 Helodus, Psamniodus and Sandalodua. Of the Petalodonts, 

 there were Ariiliodus, C/iomatodus, Ctenoptychius, Petalodus and 

 Petalorhynclms ; and of the Hybodonts, the genera Cladodus, 

 Carcharopsis and Diplodus. These Elasmobranchs were the 

 rulers of the Carboniferous open sea, and more than one hun- 

 dred species have been found in the lower part of this forma- 

 tion alone. The Ganoids, although still abundant, were of 

 smaller size, and denizens of the more shallow and confined 

 waters. The latter group of fishes was represented by true 

 LepidostidoB, ol" the genera Palceoniscus^ Amhlypterus^ Platyso- 

 mus and Eurylepis. Other genera are, Bhizodus, Megalichthys^ 

 Ctenodus^ JEdesius, Orodus, Cienacanihus, Gyracanthus, and Ciela- 

 canthus. Most of these genera occur also in Europe. 



From the Permian rocks of America, no vertebrate remains 

 are known, although in the same formation of Europe Ganoids 

 are abundant; and with them are remains of Sharks, and some 

 other fishes, the affinities of which are doubtful. The Palseo- 

 zoic fishes at present known from this country are quite as 

 numerous as those found in Europe. 



