OLD WORLD LIMPETS FAMILY PATE LL I D/E Shell conical, without distinct internal border; for gills a row of secondary branchi are substituted, set in a ring between mantle and foot; jaw and radula well developed.
A large family sub-divided upon such obscure and difficult characters as the teeth of the radula, and the branchial cordon.
Genus PATELLA, Linn.
Patella, when young, has a nautiloid shell, but it is a remark able fact that we are entirely ignorant, in this commonest of mollusks, of the transition stages which convert the nautiloid into the familiar conical shell.—Cooke.
The European limpet chooses a spot on the surface of a rock as a place of residence, and there it sinks and smooths a shallow pit exactly fitted to its shell. We are still guessing how the creature clings with a tenacity that sustains a weight of thirty pounds before the hold gives away. Back to its own place at nightfall comes the individual after ranging over the rocks to 234 The Limpets. Tent Shells feed upon minute vegetation that grows on them in patches. A peculiar noise is made by the scraping of the rock surface by the radulas of many feeding limpets. At rest this remarkable toothed tongue is coiled like a watch spring. Still louder is the rasping sound of limpets dragging their shells over wet barnacle-covered rocks, between tides.
Instead of having true gills, like those of its near relatives, this limpet has these reduced to mere stumps, and replaced by a series of gill plates, encircling the mantle. With these breathing organs the mollusks are able to remain for hours out of sea water, and to be exposed to rain without inconvenience. The oyster catcher deftly pries their shells from the rock with its case knife bill.
We shall find limpets of this genus in collections but not on our seashores. They have a wide distribution in the eastern and southern hemispheres.
The Common Tent Shell or Limpet (P. vulgata, Linn.), found from the Arctic shores to Spain, is a solid, conical shell, its peak a little in front of the centre. Ribs radiate from apex
to margin; small ribs, and still smaller strim, lie between the cardinal ones. Colour varies from grayish brown to yellowish. All are streaked or mottled, and become worn or overgrown with nullipores when old. The linings of the shells are polished and often opalescent, sometimes brighter in colouring than the exterior. Length, 1 to 2 inches.
Habitat.—Europe.
The Rusty Limpet (P. ferruginea, Gmel.) is very heavy, and deeply sculptured into rounded pillars or ridges that radiate from the apex, and make the margin deeply notched. Concentric strie cross these ridges. The back is rusty brown, shaded with white in wavy lines. The lining is white porcelain. Length, 2 to 4 inches.
The finest specimens of tent shells are found in far off tropical regions. P. longicosta from the Cape of Good Hope, has the ridges of its heavy shell prolonged into thin blades. P. granu laris has its peaked roof beset with stout prickles. The flat yellow back of P. aspera of Madeira bears a set of radiating saw toothed ridges as sharp as knife blades. P. radians from New Zealand has a flattened, almost smooth, finely mottled exterior 235 The Limpets. Tent Shells with a shell lining like smoked pearl. P. compressa from the Philippines has its thin, yellow, finely striated shells drawn in at the sides, elevating the apex.
Habitat.— Cape of Good Hope.
Limpets of this shape are often called "clowns' caps." Among the key-hole limpets genera of cap-shaped shells are also found. The "white cap" in Acmwa also has this form.