LYONSIA. 
£9 
Closely resembling Mya is Platyodon cancel - 
latus, Conr., Fig. 2, PI. XVI. This genus is 
named from its broad hinge-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 umbo. Concentric markings 
are very plain, but radial lines are faint. It is 
found abundantly in Bolinas Bay. Its color is 
white. 
Lyonsia Calif 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 localites 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 
these borers. This shows their operations are 
mechanical, and not chemical. It is now pretty 
satisfactorily ascertained that the Piddoek bores 
his hole by turning his shell back and forth, by 
means of his strong foot. The many little 
points and ridges cn the front of the shell act 
like the teeth of a file, and gradually wear away 
the hard rock. 
First, we mention a small but common borer, 
Pholadidea jpenita , 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 of the 
