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Family Acmaeidae

SHELL bowl-shaped, conical, with the apex a little in front of the middle, not spiral at any stage of development; with distinct internal border of the aperture; lining never iridescent; a free branchial plume at left above neck; radula lacks middle teeth. Mostly marine mollusks living on seaweed and rocks near shore.

Genus ACMAEA, Eschs.

The structure of a limpet may be made out with little trouble, for an animal that lives in such a shell cannot be very secretive as to the arrangements of its "in'ards." Slip a knife blade under the shell and it rolls off the rock into your hand. The branchial plume, extended at the left side of the neck when the limpet travels, is drawn in, but not concealed. The central, muscular disk is the foot, which has very remarkable tenacity when affixed to a rock face. In front the short head with its mouth and pair of tentacles appears; encircling all is the mantle border, lining the shell. From the mouth of a dead specimen draw the toothed radula, and examine the series of teeth under a good magnifier. This is the organ which rasps the algae from the rocks.

What a safe shelter is the arching roof under which this mollusk lives! Yet to breathe, the shell must be slightly lifted. This gives watchful and hungry crabs and sea birds their only chance to catch limpets unawares. They are quick to save themselves when warned. But many pay for their inattention with their lives.

Should the strongest arm endeavour

The limpet from its rock to sever,

'T is seen its loved support to clasp

With such tenacity of grasp

We wonder that such strength should dwell

In such a small and simple shell.--Wordsworth.

The Tortoise-shell Limpet (A. testudinalis, Mull.), common on the Maine and Alaskan coasts, reaches 1 1/2 inches in length, but in the British Isles it is smaller. Brown and green stripes radiate from the apex, crossing concentric circles of white and black on the gray ground in a more or less irregularly tessellated pattern. The lines of growth are rather strong, and the surface is finely sculptured with striae that cross each other. Within the aperture there is a brown and white tesselated border, then a white lining with a large owl-shaped patch of brown, the muscle scar, under the apex.

This sluggish mollusk wanders forth to feed on the soft tissues of algae. It returns to its own place on the under side of a rock after each excursion.

Var. alveus, Conr., is so thin that the checkered pattern is seen through the wall. The sides are compressed to fit the shell to the stems of seaweeds on which it lives. The elevated peak sometimes forms a forward-pointing hook.

These, like the typical tortoise-shells, are found in cold waters on both Atlantic and Pacific coasts of North America, along with forms that intergrade between them.

A. candeana, Orb., a Floridian species, has seven to nine rays of black running down the gray or buff sides, or the black spreads in more numerous, finer rays. Five lines of black cross the narrow inner border. The ovate shell is depressed; the surface cut by radiating striae. Length, 1 inch.

Habitat.--West Florida, Bahamas.

The west coast of the United States is particularly rich in species and varieties of this genus. I will not describe all of them, but select the most common and distinctive.

The Plate Limpet (A. patina, Eschs.), modified forms of which show it to be closely related to A. testudinalis, is the most characteristic limpet of the west coast. The shell is flattened, the blunt apex near the middle; the back is gray, finely tesselated with black. The flesh is white. Length, 2 inches.

Habitat.--Aleutian Islands to San Diego, Cal.

The Rough Limpet (A. scabra, Rve.) is sculptured with close, radiating, scaly ridges diversified regularly by ribs of greater size and elevation. Yellow with faint brown markings is the usual colouring. The apex is low, the slopes convex. The flesh is black.

Var. limulata, Cpr., has a black band around the apex, and a black border inside the aperture. The rest of the interior is olive green. A cap-shaped form, with coarse sculpturing is the extreme of variation in this species. Length, 1 1/2 inches.

Habitat.--California.

The Ghost Limpet (A. spectrum, Rve.) has a ghostly print of a human hand showing under the white callus that lines the shell. Outside, strong rough ridges run from the peak to the crenulated margin. They are whitish, with dots of brown filling in the depressions. Shell heavy. Length, 1 1/2 inches.

Habitat.--Sitka to Lower California.

The Mask Limpet (A. persona, Eschs.) has its beak bent forward until it is parallel with the base, and almost above the anterior edge of the shell. Behind the apex the curve is rounding. Thus the shell has almost a perfect mask form. Strong rounded ridges, with wide flat spaces between, radiate from apex to margin, crenulating the latter. The colouring varies from olive green to black, with speckles or stripes of white.