362 piiiLosornY of zoology. 



of England are jnucli more numerous than those in Scot- 

 land. In the sea at the south of England, the pilchard is 

 found in abvmdance, while it is rare in Scotland. In the 

 seas of the north of Scotland, the tusk (Gadus brosme) 

 abounds, in the south of Scotland it is very rare, and in 

 England it is unknown. 



The investigation of those revolutions which fishes have 

 experienced since the formation of the globe, is attended 

 with peculiar difficulty. The external form, on which in 

 general the specific distinction is founded, is destroyed by 

 pressure. All distinct traces of the softer parts have disap- 

 peared, and the geognost is left to draw his conclusions from 

 the form of the teeth, or the outline and structure of the 

 skeleton. Hence, the conclusions which have been drawn 

 respecting the particular species should be received with 

 caution. In the newer rock-formations, which have been 

 termed locals such as the strata at Eningen, the remains of 

 fishes have been observed, belonging to existing races, and 

 still natives of the neighbouring lakes. But, in the rocks 

 of those formations which are called tmiversal, the skele- 

 tons of fishes which have been found, in all probability, be- 

 long to species now extinct. In examining the organic re- 

 mains which we consider of this sort, it would appear, that 

 the teeth of unknown sharks are more numerous than those 

 of any other description offish. They.are found in all the 

 floetz limestones of this country, in comj)any with the an- 

 cient cameratcd shells. Vertebrae of osseous fishes are chief- 

 ly found in the strata connected with the chalk-formation, 

 seldom in those of an older date. 



Migration of Fishes. — Those fishes which enter rivers 

 for the purpose of spasming, perform their migrations an- 

 nually, but do not appear at any very precise period. Their 

 motions appear to be regulated by the condition of their gc- 



