EVOLUTION 107 



faculty of mooring themselves by a byssus and so 

 defying the waters. Mytilus — the Common Shore 

 Mussel — which also comes of a family having a long 

 pedigree, has not a particularly stout test capable of 

 resisting heavy blows, but it meets the waves with 

 its outwardly directed, sharp, wedge-shaped shell and 

 cleaves them instead ; while it does not settle, or 

 perhaps to speak more accurately, does not establish 

 itself in spots where it would be liable to damage 

 from stones thrown up by the sea. 



Most of the Bivalves, as a matter of fact, do not 

 live in exposed situations, but burrow more or less 

 deeply into soft sand or silt. Here those that do 

 not penetrate to any depth below the surface and do 

 not live in deep water beyond the reach of ground 

 swells are liable to considerable pressure from the 

 shifting of the loose material that surrounds them. 

 Hence these generally have acquired stout, more or 

 less globular shells as in Isocardia (Plate XXX., 

 Fig. 9), the Cockle, Cardtum* the Veneridse, etc. 



The disadvantage of this form of shell, of course, 

 is the amount of muscular power required to force a 

 passage with it down into the sand. A gauge of this 

 may be seen in the huge scar of the retractor pedis 

 muscle in the Veneridae, that has generally been 



* It is interesting to note that the spines on the shell of the 

 prickly species of Cardium are more pronounced on those 

 dwelling in sand than on the individuals inhabiting muddy or 

 silty sea-floors, the more shifting material demanding better 

 means of anchorage. 



