MOLLUSKS 131 
Among the most interesting of the mussels are the rock boring 
forms called Lithophagus, Fig. 92, which are common in all tropi- 
cal oceans. When young 
the shell bores into, or dis- 
solves out, a cavity for 
itself within coral rocks 
or dead coral, and there it 
remains throughout life, 
enlarging the cavity as it 
grows. It is interesting 
to notice that some of the 
species of rock boring 
mussels attach them- 
selves to the inside of 
their rocky tunnels by 
means of a_ byssus, al- 
though this can certainly 
serve no useful purpose, 
as the opening of the tun- 
nel is always too small to allow the shell to drop out. It is evi- 
dently a habit inherited from their remote free-living ancestors. 

Fig. 92; Rock-boring Mussel (Lithophagus) within 
a fragment of coral breccia. Tortugas, Florida. 

Fig. 93; RAZOR SHELL. Florida. 
The author has observed a byssus formed by Lithophagus from the 
Bahamas having a shell four and one-half inches long. 
The Razor Shell, (Pinna muricata, Fig. 95), is another inter- 
