The mantle is the part of the body wall immediately under the shell and at the edges it is curled round to form a collar.

The head bears two sensory tentacles with a tiny black eye at the base of each.

[3] T. testudinalis occurs in northern regions of both the Pacific and Northwest Atlantic Oceans and in European waters.

It is found in the neritic zone from low water mark down to a depth of about fifty metres.

It is usually found on stones and boulders, especially those encrusted with red crustose algae such as Lithothamnion.