Rostrum, the beak; the extension of the shell, in which the canal is situated.
Rotund, round, circular, spherical.
Rudimentary, the commencement or first elements of anything; generally applied to the indistinct teeth of shells.
Rufous, of a reddish colour.
Rugose, rugged, full of wrinkles.
S.
Sanguinaceous, of a blood colour, or resembling blood.
Scabrous, rough, rugged, harsh, or like a file.
Scalloped, indented at the edges.
Scrobiculate, pitted, having the surface covered with hollows.