1 88 HISTORICAL PALEONTOLOGY. 



apparently disappeared ; and the principal types of the Car- 

 boniferous belong to the groups respectively represented at 

 the present day by the Gar pike (Lepidosteus) of the North 

 American lakes, and the Polyptcrus of the rivers of Africa. Of 

 the former, the genera Palcewiiscus and Amblyptcrus (fig. 130), 



croptenis. Carboniferous. 



with their small rhomboidal and enamelled scales, and their 

 strongly unsymmetrical tails, are perhaps the most abundant. 

 Of the latter, the most important are species belonging to 

 the genera Megalichthys and Rhizodus, comprising large fishes, 

 with rhomboidal scales, unsymmetrical (" heterocercal ") tails, 

 and powerful conical teeth. These fishes are sometimes said 

 to be " sauroid," from their presenting some Reptilian features 

 in their organisation, and they must have been the scourges 

 of the Carboniferous seas. The remains of Placoid fishes in 

 the Carboniferous strata are very numerous, but consist wholly 

 of teeth and fin-spines, referable to forms more or less closely 

 allied to our existing Port Jackson Sharks, Dog-fishes, and 

 Ravs. The teeth are of very various shapes and sizes, some 

 with sharp, cutting edges (Petalodus, Cladodus, &c.) ; others in 

 the form of broad crushing plates, adapted, like the teeth of the 

 existing Port Jackson Shark (Cestracion Philippi), for breaking 

 down the hard shells of Molluscs and Crustaceans. Amongst 

 the many kinds of these latter, the teeth of Psammodus and 

 Cochliodiis (fig. 131) may be mentioned as specially charac- 

 teristic. The fin-spines are mostly similar to those so common 

 in the Devonian deposits, consisting of hollow defensive spines 

 implanted in front of the pectoral or other fins, usually slightly 

 curved, often superficially ribbed or sculptured, and not un- 

 commonly serrated or toothed. The genera Cte?iacanthus, 

 Gyracanthus, HomacantJms, &c., have been founded for the 

 reception of these defensive weapons, some of which indicate 

 fishes of great size and predaceous habits. 



