154 CYCLOBRANCHIATA.—PATELLADZA. 
genus, on the surface of which it forms a flat disk, 
exactly agreeing in size with the circumference of 
its own shell. ‘To form these depressed disks, (of 
which there are so generally two on each larger 
Patella, one on each side of the apex, as almost to 
form a character of the species,) and to assist in 
the increase of its size, the animal appears also to 
absorb the coralline or other similar substances 
with which the larger shells are abundantly 
covered.* 
But we need not wander to the southern hemi- 
sphere for illustrations of this power. The most 
familiar shell-fish of our shores, the common Lim- 
pet (P. vulgata), will afford one equally good. 
ho has not seen the oval pits, sometimes but 
just discernible, at others sunk to the depth of an 
eighth of an inch or more, on the rocks of our 
coast, each accurately corresponding in shape and 
COMMON LIMPET. 
dimensions with a Limpet which inhabits it? I 
have wondered at them many times, not being then 
aware of the habits which have been ascribed to 
these animals, of wandering away from these pits, 
(which they have chosen for a home,) and of re- 
turning to them regularly again. 
* Gray, in Phil. Trans. 1833. 
