FISHES m MAGNESIAN LIMESTONE. 213 



contraction, on putrid vegetables and animal substances at the 

 bottom ; hence they have constant occasion to keep their 

 bodies in the same inclined position as the extinct fossil 

 Fishes, whose feeble brush-like teeth show that they also fed 

 on soft substances in similar situations.* 



The Sharks employ their tail in another peculiar manner, 

 to turn their body in order to bring the mouth, which is 

 placed downwards beneath the head, into contact with their 

 prey. We find an important provision in every animal to 

 give a position of ease and activity to the head during the 

 operation of feeding.f 



Fishes of the Magnesian Limestone, or Zechstein. 



The Fishes of the Zechstein at Mansfeld and Eisleben 

 have been long known, and are common in all collections ; 

 figures of many species are given by M. Agassiz. Exam- 

 ples of the Fishes of the Magnesian limestone of the north 

 of England, are described and figured by Professor Sedg- 

 wick, in the Geol. Trans, of London, (2d Series, Vol. iii. p. 

 117, and PI. 8, 9, 10.) He states in this paper (p. 99,) that 

 the occurrence of certain Corals and Encrinites, and several 

 species of Producta, Area, Terebratula, Spirifier, &c. shows 

 that the Magnesian limestone is more nearly allied in its 



♦ At the siege of Silistria, the Sturgeons of the Danube were observed to 

 feed voraciously on the putrid bodies of the Turks and Russian soldiers that 

 were cast into that river. 



•j- This remarkable elongation of the superior lobe of the tail is found in 

 every bony Fish of strata anterior to and including the Magnesian lime- 

 stone ; but in strata above this limestone the tail is usually regular and sym- 

 metrical. In certain bony Fishes of tiie secondary period, the upper lobe of 

 the tail is partly covered with scales, but without vertebrae. The bodies of 

 all these Fishes also have an integument of rhomboidal body scales, covered! 

 with enamel. 



No species of Fish has been found common to the Carboniferous group, 

 and to the Zechstein or Magnesian limestone; but certain genera occur in 

 both, e. g. the genus Palseoniscus and Polypterus. 



