CHAPTER III 

 GEOLOGICAL HISTORY 



THE Mollusca made their appearance very early 

 in the world's history, and, as might be expected, 

 on the whole the more generalized forms preceded 

 the more specialized. It has, of course, to be borne 

 in mind that many of the conclusions here epitomized 

 have been drawn of necessity from shell characters 

 alone, and are consequently liable to modification 

 with possible advance of knowledge. 



In the oldest f ossiferous beds, belonging to the 

 Lower Cambrian epoch, only a few representatives 

 have been found. These consist of some Limpet- 

 like shells that have been referred in part (Scenella) 

 to the Docoglossa, and in part (Stenotheca, Platyceras) 

 to the Tsenioglossa. These last, however, it has 

 been suggested are, instead, descendants of the primi- 

 tive Mollusca (Prostreptoneura) that gave rise to the 

 Gastropod branch. With these is a turreted, con- 

 volute shell (Rhaphistoma) representing the older, or 

 two-gilled, section of the Rhipidoglossa, and the 



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