Limpets. 107 
rows, four deep, of sharp hooked teeth, and between 
each row are placed two others, which have three 
points, and are set in a slanting position. The use 
of this arrangement we cannot at present deter- 
mine, but no doubt it has a perfect adaptation to 
the wants of the animal. . 
There are shell-fish called Key-hole Limpets, 
which belong to the genus Fissurellide, from fissura, 
a cleft or slip, from whence comes also fissure. All 
the members of this genus are distinguished by the 
aperture at the top of the shell, shaped like a key- 
hole, which is situated exactly over the breathing 
organs, and serves as a channel for the water neces- 
sary for respiration. 
Frequently upon the fronds of the large olive 
sea-weeds may be found a tiny shell shaped some- 
thing like that of the Common Limpet. It is of an 
olive-green colour, with blue streaks, and is called, 
from its clearness, the Pellucid Limpet (P. pellucida), 
There is also another much like it in appearance, 
which naturalists call P. levis. To the labours of 
these little mollusks, according to Dr. Harvey, may 
be attributed the destruction of the gigantic Alew 
(sea-weed). Hating into the lower part of the 
stems, and destroying the branches of the roots, 
they so far weaken the base that it becomes unable 
