462 



MAGNESIAN LIMESTONE. 



[Oh. XXIII. 



all the more ancient formations. It characterizes the earlier periods 

 of the earth's history, whereas in the secondary strata, or those newer 

 than the Permian, 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 mono- 

 graph 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 characteristically marked as to indicate the genus, and 

 sometimes even the particular species. They are often scattered 

 through the beds singly, and may be useful to a geologist in deter- 

 mining the age of the rock. 



Fig. 502. 



Scales of fish. Magnesian Limestone. 

 Fig. 503. Fig. 504. 



Fig. 505. 



Fig. 502. Palceoniscus comptus, Agassiz. Scale, magnified. Marl-slate. 



Fig. 503. JPalceoniscus elegans, Sedg. Under surface of scale, magnified. Marl-slate. 



Fig. 504. Palceoniscus glaphyrus, Ag. Under surface of scale, magnified. Marl-slate. 



Fig. 505. Ccelacanthus granulatus, Ag. Granulated surface of scale, magnified. Marl-slata 



Fte. 506. 



Fig. 507. 



Pygopterus mandibularis, Ag. Marl-slate. 

 a. Outside of scale, magnified. 

 &. Under surface of same. 



Acrolepis SedgwicMi, Ag. 



Outside of scale, magnified. 



Marl-slate. 



The inferior sandstones (No. 6, Tab., p. 458), which lie beneath the 

 marl-slate, consist of sandstone and sand, separating the magnesian 

 limestone from the coal, in Yorkshire and Durham. In some in- 

 stances, red marl and gypsum have been found associated with these 

 beds. They have been classed with the magnesian limestone by Pro- 

 fessor Sedgwick, as being nearly coextensive with it in geographical 

 range, though their relations are very obscure.. In some regions we 

 find it stated that the imbedded plants are all specifically identical 

 with those of the carboniferous series ; and, if so, they probably be- 

 long to that epoch; for the true Permian flora appears, from the 

 researches, of MM. Murchison and de Yerneuil in Eussia, and of MM. 

 Geinitz and Yon Gutbier in Saxony, to be, with few exceptions, dis- 

 tinct from that of the coal (see p. 463). 



