414 



VEGETATION OF THE CARBONIFEROUS PERIOD 



IS 



The zoologist is not equally hampered with the three classes of diffi 

 culties described above. Take the mollusca for instance ; there 

 hardly an instance of two shells, identical in appearance, yet belonging 

 to different genera of that order, and only distinguishable by the animal 

 they contain, or by one organ of that animal ; far less of four genera of 

 shells so circumstanced, and each the type, not of a genus, but of 

 groups — groups, too, differing in geographical distribution as well as in 

 their most important characters. 



Fig. 10. 



Outline again, a fallacious guide to the determination especially of 

 generic affinity amongst plants, is of eminent importance in the animal 

 kingdom, where the habits of the different groups, all locomotive, de- 

 mand peculiar modifications of the hard parts, whether of the external 

 or internal skeleton. 



Surface. — The presence or absence of hairs or scales on the surface 

 of the fronds, and sometimes of glands also, affords characters of the 

 greatest constancy for distinguishing species, being often available when 

 the outline of the fern is so variable, that by it alone the species cannot 

 be traced. The larger and most characteristic hairs or scales occur 



