92 THE GEOLOGICAL EVIDENCES 



common crown-conch of the Gulf by several 

 well-marked characters, of which the deficiency 

 in the number of tubercles to the different 

 whorls, and the horizontal position occupied 

 by them, are especially apparent. The tuber- 

 cles are also more compact, and do not 

 show the foliaceous or scaly character which 

 they exhibit in the living species. But while 

 these differences in structure readily serve to 

 distinguish the typical or most abundant forms 

 of the two species, they in a measure fail when 

 some of the less typical forms are taken by 

 way of comparison. Thus, a tendency toward 

 increase or duplication in the number of 

 tubercles is here and there apparent in the fossil 

 form, while, per contra, a tendency toward 

 deficiency is not exactly rare in the recent 

 species. Similarly, the tubercles or nodes of the 

 fossil, which in the typical forms stand out 

 nearly horizontally from the shoulder of the 

 shell, or have but a moderate inclination, are 

 occasionally more nearly directed in the position 



