ORGANIC i:\()LUTI()X 



O " '» 



pond slu'll similar to the ])lanorbis, wliich is so common at 

 the ])rcscnt lime. 



Fig. 104 shows some of these transformations, the finer 



Fig. 104. — Planorbis Shells from Stcinheim. (After Hyatt.) 



gradations being omittecL The shells from these two sources 

 bear directly u};on the (juestion of wliellier or not species have 

 held rit^idh' to their original form. 



-J 



