7O THE DOCTRINE OF DESCENT. 
which they have in common with several of the crabs 
now inhabiting shallow waters and coasts, and likewise 
their general habit, allow us to infer that they also were 
denizens of coasts. 
The Molluscs were mainly represented by Brachiopoda 
and Cephalopoda. However, as Bivalves and Gaster- 
FIG. 10 
opoda were also in existence, the appearance of this, the 
most ancient molluscous fauna known, differs from the 
Present one only in its numerical proportions, and in the 
