334 PATELLIDA. 
out, by degrees, with its muscular foot a cavity of a greater or 
less depth. Specimens are not unfrequently found, on impure 
limestone, which are constricted or indented at the edges, in 
consequence of the excavation having been hindered by the 
greater hardness of one side of the spot occupied by these 
limpets. The animal feeds on small, delicate sea-weeds of a 
foliaceous kind, as well as on Melobesia polymorpha, that encrust 
the rocks at low-water, by means of its long tongue, which is 
coiled spirally, like the main-spring of a watch, set around with 
cogs. This instrument is thrust out from side to side, and when 
charged with food, it is withdrawn into the stomach, unloaded, 
and again put forth. The mark left on the face of a rock, coated 
with a film of the fine sea-weed mentioned above, by a limpet 
after grazing, resembles the track of a sea-worm ; indeed, a late 
eminent geologist had a large slab thus marked cut out of the 
rock, and sent to him with great care,in order to publish the 
supposed discovery of a new Annelidan ichnolite in the old red 
sandstone. Fortunately, the mistake was pointed out to him 
before he proceeded further. Each limpet appears to have its 
own feeding-ground or pasturage; its tracks are sometimes 
numerous, and deviate in different directions. Mr. Peach has 
ascertained that it does not retire in the winter to deeper water, 
on the coast of Caithness, and that it always returns home before 
the ebbing tide leaves it dry. Its firm adhesion to the rock is 
extraordinary. In order to test the strength of its tenacity, 
Reaumur suspended a weight of 28 to 30 lbs. from the shell of 
a limpet attached to a stone. This weight it sustained for some 
seconds; less weight failed to overcome its resistance. He 
attributes the adhesive force not to muscular action, but to an 
invisible glue which exudes from the granulated base or sole of 
the foot. It may be also caused by the adaptation of the surface 
of this part of the animal to the frequent, although often minute, 
inequalities of the stone, although the glutinous and viscous 
fluid, which is secreted by numerous glands in the foot, appears 
to be the principal agent.”—JEFFREYS, British Conchology. 
PATINELLA, Dall. Shell solid, porcellanous, with an erect sub- 
central apex. Foot bordered by a a scalloped frill, interrupted 
only in front. P. Magellanica, Gmel. (1xxxiii, 23). 
NACELLA, Schum. Shell with the apex submarginal, anterior ; 
pellucid, thin or corneous. Animal with the ‘branchial cordon 
complete before and behind, the laminw persistent but dimin- 
ishing in size before the head; sides of the foot with scalloped 
lappets. P. cymbularia, Lam. (lxxxiii, 24). 
HELCION, Montfort. Shell ovate, radiately ribbed ‘pectinated) ; 
apex anterior, submarginal; aperture ovate, edge crenated. 
Branchial cordon interrupted i in front; sides of the “foot smooth. 
P. pectinata, Linn. (Ixxxiv; 50).. 
