Ch. XXIII.] FOSSIL FISH OF PERMIAN" MARL-SLATE. 



353 



of fossil fisli of the genera Palceoniscus^ Pygopterus, Coelacanthus, and 

 Platysomus, genera which are all found in the coal-measures of the car- 

 boniferous epoch, and which, therefore, says Mr. King, probably lived at 

 no great distance from the shore. But the Permian species are peculiar, 

 and, for the most part, identical with those found in the marl-slate or 

 copper-slate of Thuringia. 



Fig. 450. 



Eestorcd outline of a fish of the genus Falmo7iiscus, Agass. 

 Palceothriss urn, Blainy ille. 



The Palceoniscus above mentioned belongs to that division of fishes 

 which M. Agassiz has called " Heterocercal," which have their tails une- 

 qually bilobate, like the recent shark and sturgeon, and the vertebral 

 column running along the upper caudal lobe. (See fig. 451.) The 

 " Homocercal" fish, which comprise almost all the 8000 species at present 



Fig. 451. 



Fig. 452. 



Shark. 

 Reterocereal. 



Shad. (Clupea, ILerring tribe.) 

 Homocercal. 



known in the living creation, have the tail-fin either single or eq-ually 

 divided ; and the vertebral column stops short, and is not prolonged 

 into either lobe. (See fig. 451.) 



ISTow it is a singular fact, first pointed out by Agassiz, that the heter- 

 ocercal form, which is confined to a small number of genera in the exist- 

 ing creation, is universal in the Magnesian limestone, and all the more 

 ancient formations. It characterizes the earlier periods of the earth's 

 history, when the organization of fishes made a greater aj^proach to that 

 of saurian reptiles than at later epochs. In all the strata above the 

 Magnesian limestone the homocercal tail predominates. 



A full description has been given by Sir Philip Egerton of the species 

 of fish characteristic of the marl-slate in Prof King's monograph before 

 referred to, where figures of the ichthyolites which are very entire and 

 well preserved, will be found. Even a single scale is usually so charac- 

 teristically marked as to indicate the genus, and sometimes even the par- 



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