90 THE GEOLOGICAL EVIDENCES 



but these differences are so well marked as to 

 obscure at first sight the relationship. In the 

 majority of the fossil forms the wing is more 

 or less evenly crescentic in outline, whereas in 

 the recent species it is markedly quadrangular 

 in its upper moiety, so much so that in extreme 

 specimens the outline is wholly different from 

 that seen in the fossil. But Strombus Leidyi 

 shows a pronounced tendency to vary in the 

 direction of 5. accipitrinus, and conversely the 

 latter, in this regard, seems to vary equally in 

 the direction of the former, so that we have 

 an almost perfect gradation established between 

 the extreme wing-structures seen in the one 

 species and the other, or between the almost 

 perfectly crescentic outline and that which 

 exhibits the greatest quadrangulation. In a 

 similar manner the very prominent tubercles 

 seen in the recent species, which are represented 

 by elongated nodes in the fossil, are more or 

 less lost in some individuals, although they at 

 all times appear more prominent than in the 



