28 CONCHOLOGIST’S COMPANION. 
observe a very different mode of structure from that 
of the solitary Pholas, and this distinction bears an 
obvious reference to a very different mode of life. 
The exterior admits of considerable variety in its 
colouring and shape, but the usual outline is conical, 
or hemispherical, which form is acquired by a 
number of immoveable valves, placed perpendicu- 
larly on a base, broad at the lower margin, and 
gradually tapering towards the summit, where they 
are closed by a moveable lid, or covering, formed 
of smaller ones. This construction furnishes, in 
my opinion, a sufficient clue to the general habits of 
the animal. 
These extraordinary shell-fish are never found 
detached from other substances. They adhere by 
the base or stalk, not only to rocks and stones, but 
even to marine animals, such as the whale and 
turtle. They are also found on vessels, and increase so 
rapidly in size and number, as sometimes to impede 
their progress in sailing. And yet, though generally 
fixed to other bodies by the base, in some few 
instances, as those of the L. Scalpellum; L. Anseri- 
fera; L. Anatifera; or Knife-like, Striated, and Duck 
Barnacles, with several varieties emanating from them, 
they are attached by a stem. These appendages 
differ considerably both in character and substance; 
they occasionally resemble a smooth film-like tube, 
