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 voung 

 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 

 sj^ecies of rock boring 

 mussels attach them- 

 selves to the inside of 

 their rocky tunnels bv 

 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. (j3; liock-boriutt Mussel ( Lit/iophagus) witliin 

 a fragment of coral breccia. Tortugas, Florida. 



Fig. 93; RAZOK SHELL. Florida. 



The author has observed a byssus formed by Lithophagus from the 

 Bahamas having a sliell four and one-half inches long. 



The Razor Shell, f Pinna muricata, Firj. 93), is another inter- 



