SPECIAL GEOLOGY. 433 



Of bhe two fish above named, lie observes, that the former 

 perform the office of scavengers, to clear the water of 

 impurities, as they have no teeth, but feed at the bottom, 

 on putrid animal and vegetable substances, by means of a 

 soft, leather-like mouth, capable of protrusion and retraction; 

 hence they have occasion to keep their bodies in the same 

 inclined position as the extinct fossil fishes of this formation, 

 the Palfeoniscus, whose small arid numerous teeth show that 

 they also fed on like substances in similar situations. 



EXERCISES 



ON THE TB1ASIC AND PERMIAN GROUPS. 



1. Describe the authors and collections. 



2. Point out the general features of these groups. 



3. Mention their principal geographical distribution. 



4. Trace their occurrence on the map. 



5. Name the counties where they are developed. 



6. State the division of the strata. 



7. Mention their mineral composition. 



8. Point out those which contain organic remains. 



9. State the presumed cause of' the paucity of fossils 

 in some rocks of this group. 



10. Mention the mineral contents of the strata. 



11. State the localities of their occurrence. 



12. Mention any general facts of interest or importance 

 connected with these deposits, and the questions and 

 answers illustrative of their history. 



