Shell conical, oval, the apex a little in front of the middle; posterior slope slightly convex, other slopes straight; surface more or less distinctly, finely radiately striated; color yellowish-gray, with numerous blackish-brown stripes, generally broken into a coarse network, or tessellated pattern.
Inside white, with a large dark brown central area, the border tessellated brown and white. Length (up to) 38, breadth 28, alt. 13 mill.

As Collisella tessulata
Shell approximately conical; without marginal slit, apical hole or internal septum. Apex tilted forwards, one third of shell length behind anterior end. Surface rather smooth. Irregular chocolate brown marks radiate from apex; pale sectors posterolaterally. Inner surface with brown and white margin, central area mainly chocolate. Mantle skirt green, edged with tentacles but no pallial gills; one ctenidium in nuchal cavity. Foot without operculum and epipodial tentacles. A minute spiral coil may be seen at the apex of shells about 1 mm long, but it is later lost. Many fine ridges radiate from the apex and are crossed by others parallel to the aperture edge. The aperture is oval, sometimes narrower anteriorly. The shell is basically cream or greenish, its radial bands of brown often bifurcating towards the edge. The central brown mark internally has usually a pale patch at the head end. Up to 20mm long, 14 mm broad, 10mm high; intertidal animals are commonly about half these dimensions.
The head has a broad snout with two tentacles each with a swollen base on the dorsal side of which is a small eye. The lips round the mouth are not drawn out into posterolateral processes as they are in Acmaea virginea. The mantle edge carries a double row of short tentacles. The foot has a large, oval sole. Cream with brown round the mouth; the red buccal mass shows by transparency.

Atlantic specimens of this well-known shell, although very variable in size and coloration, are readily distinguished from the other forms, the only considerable divergence being found in var. alveus. Specimens from the Aleutian Is., according to Dr. Dall, completely bridge the gap between testudinalis and patina. I have retained the latter separate, simply as a matter of convenience; but I do not doubt that it must be regarded as a geographic race of testudinalis. American specimens are generally larger than European.

North Atlantic and Arctic Oceans, southeast to the English Channel, southwest to Long Island Sound; North Pacific from Sitka (and Yesso) to the Arctic Ocean.

C. tessulata is common on small stones that are firmly fixed on rocky shores and bear red weeds, on which it feeds. It occurs from L. W.S.T. (M.H.W.N.T. in pools) to depths of 50m. In the British Isles it is found only north of a line from Dublin to Anglesey in the Irish Sea and north of Yorkshire in the North Sea; not on northern or western Irish coasts, but there is a recent report of its occurrence on the French coast south of the Channel Islands. The species is circumpolar in its distribution.

Eggs are laid in spring and early summer, fastened to a firm substratum in a mucous sheet in which they are embedded one layer deep. They hatch rapidly as trochophore larvae which metamorphose and settle after a short free life.