Ch. XXin.] FOSSIL FISH OF FERMIAN MARL-SLATE. 



461 



type. Some of this same tribe of shells, such as Athyris Roissyi, 

 allied to Terebratida, are specifically the same as fossils of the car- 

 boniferous rocks. Avicida, Area, and Schizodus (see above, fig. 492), 

 and other lamellibranchiate bibalves, are abundant, but spiral uni- 

 valves are very rare. 



The compact limestone (No. 4) also contains organic remains, espe- 

 cially bryozoa, and is intimately connected with the preceding. Be- 

 neath it lies the marl-slate (No. 5), which consists of hard, calcareous 

 shales, marl-slate, and thin-bedded limestones. At East ThicMey, in 

 Durham, where it is thirty feet thick, this slate has yielded many fine 

 specimens of fossil fish of the genera Palceoniscus, Pygopterus, Ccela- 

 canthus, and Platysomus, genera which are all found in the coal- 

 measures of the Carboniferous 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. 499. 



Ecstored outline of a fish of the genus Palceoniscus, Agass. 



Palceothrissum, Blainville. 



The PalcBoniscus above mentioned belongs to that division of fishes 

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

 unequally bilobate, like the recent shark and sturgeon, and the verte- 

 bral column running along the upper caudal lobe. (See fig. 500.) 



Fig. 500. 



Fig. 501. 



Shark. 

 Heterocercal. 



Shad. (Chipea. Herring tribe.) 

 Homocercal. 



The " Homocercal " fish, which comprise almost all the 9000 species 

 at. present known in the living creation, have the tail-fin either single 

 or equally divided ; and the vertebral column stops short, and is not 

 prolonged into either lobe. (See fig. 501.) 



Now it is a singular fact, first pointed out by Agassiz, that the 

 heterocercal form, which is confined to a small number of genera in 

 the existing creation, is universal in the magnesian limestone, and 



