CHAPTER III 
GEOLOGICAL HISTORY 
i ips 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 fossiliferous 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 Tzenioglossa. 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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