LYONSIA. 9 



Closely resembling My a is Platyodon cancel- 

 latus, Conr., Fig. 2, PL XVI. This genus is 

 named fr^iri its broad h in ore-tooth, which is not 

 equal, however, to that of Mya. The hinge is 

 near the posterior extremity of the shell, which 

 is bulged near the um bo. Concentric markings 

 are very plain, but radial lines are faint. It is 

 found abundantly in Bolinas Bay. Its color is 

 white. 



Lyonsia Cal!/ornica, Conr., is a delicate little 

 shell, an inch long, which is sometimes found 

 washed up on the shores of San Francisco Bay, 

 as well as at other local ites up and down the 

 coast. It is oblong, bulged at one end, and 

 pearly within. The outer coat, which shows 

 many concentric striae, is easily rubbed off, 

 showing the pearly layers. 



We come, lastly, to the boring shells, or Pid- 

 docks. These mollusks have the power of bor- 

 ing holes in rocks and hardened clay. In the 

 cabinet of the California Academy of Sciences, 

 may be seen a piece of brick pierced by one of 

 thesa borers. This shows their operations are 

 mechanical, and not chemical. It is now pretty 

 satisfactorily ascertained that the Piddock bores 

 his hole by turning his shell back and forth, by 

 means of his strong foot. The many little 

 points and ridges en the front of the shell act 

 like the teeth of a tile, and gradually wear away 

 the hard rock. 



First, we mention a small but common borer, 

 Pholadidea penita, Conr., Fig. 3, PI. XVI. It 

 is seldom more than an inch and a half in length. 

 When young, the part of the shell in front ol the 



