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, Cardium,* the Veneride, 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 Veneridz, 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, 
