GEOLOGICAL DISTRIBUTION. 197 



lites. In the ironstones they frequently form the nuclei of 

 nodules, as in the Devonian. 



Of Chondropterygians the spines of Onchus and others 

 stUl occur, with the addition of teeth indicative of the exist- 

 ence of fishes allied to the Cestracion-type (Cochliodus, Fsam- 

 modus) : a type which henceforth plays an important part in 

 the composition of the extinct marine fish faunae. Another 

 extinct Selachian family, that of Hybodontes, makes its 

 appearance, but is known from the teeth only. 



Of the Ganoid fishes, the family Falwoniscidcc (Traquair) is 

 numerously represented ; others are Coelacanths (Ccelacanthus, 

 Rhizodus), and Saurodipteridm {Metjaliclithys). None of these 

 fishes have an ossified vertebral column, but in some {Megal- 

 ichthys) the outer surface of the vertebrae is ossified into a 

 rino' ; the termination of their tail is heterocercal. The car- 

 boniferous Uronemus and the Devonian Phanero])leuron are 

 probably generically the same ; and the Devonian Dipnoi are 

 continued as, and well represented by, Ctenodus. 



The fishes of the Permian group are very similar to those 

 of the Carboniferous. A type which in the latter was but 

 very scantily represented, namely the Platysomida: , is much 

 developed. They were deep-bodied fish, covered with hard 

 rhomboid scales possessing a strong' anterior rib, and provided 

 with a heterocercal caudal, long dorsal and anal, short non- 

 lobate paired fins (when present), and branchiostegals. The 

 Pala:oniscid(B are represented by many species of Palmoniscus, 

 PygopteriLS and Acrolcpis, and Cestracionts by Janassa and 

 Stropliodus. 



The passage from the Palaeozoic into the Mcsozoic era is 

 not indicated by any marked change as far as fishes are con- 

 cerned. The more remarkable forms of the Trias are Shark- 

 like fishes represented by ichthyodorulithes like Nemacanthus, 

 Liacantlms, and Hyhodus ; and Cestracionts represented by 



