LIMPETS. 153 
The doubts occasionally cast on what are received 
as established truths in science, though they may 
seem to unsettle our knowledge, and give a cha- 
racter of vagueness to it, must not be considered as 
inimical either to its progress or its solidity; not 
even when, as in the case just cited, they are found 
to be without foundation. They give rise to new 
and more careful examinations; to extensive com- 
us of species with species, or of fact with 
act; to satisfactory inductions of principles from 
observations ; and often to the discovery of laws 
before unsuspected. 
GENUS PATELLA. 
As the family consists but of this single genus, 
the characters already enumerated need not be 
repeated. ‘The species are numerous, widely dis- 
tributed over the globe, scarcely any sea being 
destitute of some, with the exception of the Arctic 
Regions, where none have been observed by voy- 
agers. As usual, the largest species are found in 
the seas of the tropics. Deshayes in his Tables 
enumerates 104 living species, and ten fossil ; 
several others have becn added by subsequent 
naturalists, but as the genus is peculiarly liable to 
variation in the form, colour, and surface of the 
shell, it is very likely that many of these de- 
scribed species are merely varieties. 
The animals of this genus have the power of 
wearing away, or of absorbing the surface of other 
shells, or of the rocks to which they adhere, and of 
thus forming sunken pits or depressions on them. 
The Patella cochlear of the Cape of Good Hope, is 
often found attached to a large species of the same 
