334 PATELLID^. 



out, by degrees, with its muscular foot a cavity of a greater or 

 less depth. Specimens are not unfrequentl}^ 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 charged with food, it is withdrawn into the stomach, unloaded, 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 flue 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 pastiirage ; its tracks are sometimes 

 numerous, and deviate in diiferent 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 Gonchology. 



PATiNELLA, Dall. Shell solid, porcellanous, with an erect sub- 

 central apex. Foot bordered by a scalloped frill, interrupted 

 only in front. P. MageUanica, Gmel. (Ixxxiii, 23). 



NACELLA, Schum. Shell with the apex subraarginal, anterior ; 

 pellucid, thin or corneous. Animal with the branchial cordon 

 complete before and behind, the- laminae persistent but dimin- 

 ishing in size befoi-e the head; sides of the foot with scalloped 

 lappets,, P. cymhularia, Lam. (Ixxxiii, 24). 



HELCION, Montfort. Shell ovate, radiately ribbed ^pectinated) ; 

 apex anterior, submarginal ; aperture ovate, edge crenated. 

 Branchial cordon interrupted in front ; sides of the foot smooth. 

 P. pectinata, Linn. (lxsxiv,,50). 



