232 THE WORLD OF THE SEA. 
vegetation, or on the rocks; so firmly do they adhere, that a 
patella has been known to resist a force of 150 lbs. before it could 
be pulled from its hold. There are others again which burrow 
holes for themselves in the sand of the sea-bed.. The Watica mzlle- 
puncta excavates the sand with its dilated foot, cutting a groove 
OLIVA PERUVIANA. 
as it proceeds; thus it enters the earth by a decline. Its mantle is 
so large that it covers its shell, and thus its feelers and respiratory 
organs are protected; and, at the same time, its shell is preserved 
from being rubbed by the walls of the channel in which it moves. 
The marine univalves exhibit the greatest diversity in their 
shape and colours. So beautiful are most of them that they are 
THE ORANGE PTEROCERA. 
(Pterocera aurantia. 
used for ornamenting our rooms; and their colours have again and 
again emulated the painter to transfer them to his canvas. All 
the shells—or very nearly all—are covered with a horny epidermis, 
which protects their polished surface and their brilliant tints. 
This covering is found of every shade; the most common is brown, 
the rarest green, 
Some shells, instead of being of a uniform colour, are striped, 
or mottled, or spotted, or traced with curious designs and unread- 
