Shell solid, reaching a large size, up to 90 mm. (3,5 inches) in length, but most adults about two thirds that size; broadly ovate, of low rounded profile, with the apex at about the anterior third; margin simple and smooth. External sculpture consisting of weak to obsolete, low, narrow, smooth radial riblets. Colour of exterior dark greenish brown, with a radiate pattern in dark brown, within the shell substance, and apparent only when the shell is held to the light; the pattern may consist of radial streaks, joined across in a netted pattern, or it may consist of bold chevrons; internally the shell is bluish silvery, with the large elongated spatula greyish white to yellowish brown; the margin of the shell has a continuous border in dark-brown, with terminal blotches from the internal radial pattern which also shows through faintly, back almost to the spatula. Radula: All teeth long and narrow; paired centrals each with a long slender, erect, simple-pointed cusp; paired laterals larger than the centrals, each with the cusp moderately curved, ending in a sharp point, and with a weak denticle on each side, at the base of the cusp, and at a point just above middle height of the whole tooth; marginals 3, outer two more or less fused basally, and with a rudimentary cusp on the middle one only. Length 40-90 mm; Width 40-75 mm; Height 14-33 mm.

This is a large, solid, broadly ovate, and rather depressed Cellana, externally rather smooth, of brownish slate colour, radially patterned in darker brown, and bluish silvery within. This widely distributed Indo-Pacific species is almost invariably associated with dark volcanic rock, and occurs near and below low tide in exposed situations. Pilsbry's Helcioniscus rota var. discrepans proves to be a synonym of C. testudinaria. The type material consists of two undersized, badly worn examples from unknown locality. Another synonym is Dunker's insignis which Fraunfeld claimed as coming from the Cape of Good Hope, but almost certainly incorrectly. Fraunfelds figure looks like C. testudinaria, but there is no other record of the species from that area. Two related smaller species, also with a preference for dark volcanic rock, extend the range of the testudinaria type of Cellana, to the westward in C. garconi of the Madagascar-Reunion area, and to the eastward in C. vitiensis, the latter appar-ently being restricted to the Fiji Islands.

Andaman Islands to the Ryukyu Islands and to New Caledonia and North Queensland.