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214 American Seashells
length), strongly indented suture, and rugose sculpture with a row of double,
strong nodules on the shoulder of the whorls. M. D. Burkenroad (1931) has
given a long account of the biology of this oyster pest.
Thais rustica Lamarck Rustic Rock-shell
Southeast Florida and the West Indies. Bermuda.
Plate 25f
I /4 inches in length, irregularly sculptured with 2 spiral rows of blunt
spines, one on the shoulder, the other at the center of the body whorl. Color
dirty-gray to dull mottled brown. Interior of aperture whitish but generally
margined with spots of dark-brown along the outer lip. Parietal wall glossy-
white. This species is smaller and more nodulose than floridana and always
has a white aperture. Erroneously called Thais imdata Lamarck which is an
Indo-Pacific species.
Three confusing species are often found together in southern Florida.
The young of Canthariis tinctits (see p. 233) has the lower third of its white
columella turned away (to the left) 20 degrees from the axis of the shell, and
its early whorls in the spire are not shouldered. Thais haemastoina floridana
is characterized by its almost straight, cream to orange columella, and by the
numerous raised cream or white spiral ridges on the inside of the outer lip.
Thais rustica has a stouter, white columella which is slightly twisted and
purple-stained at the lower, inner corner. Its outer lip teeth occur in groups
of 2 or 3 and near the edge of the lip are stained by heavy blotches of purple-
brown.
Subgenus Mancinella Link 1807
Thais deltoidea Lamarck Deltoid Rock-shell
Jupiter Inlet, Florida, to the West Indies. Bermuda.
Plate 25b, k
I to 2 inches in length, heavy, and coarsely sculptured with two spiral
rows of large, blunt spines. Parietal wall tinted with lavender, mauve or rose.
Interior of aperture glossy-white. Exterior grayish white with mottlings of
black or dull brown. Columella with a small but distinct ridge at the base
which forms the margin of the siphonal canal. This is an abundant species
where intertidal rocks are exposed to the ocean surf.
Subgenus Polytropa Swainson 1840
Thais lapillus Linne Atlantic Dogwinkle
Southern Labrador to New York. Norway to Portugal.
Plate 2 5g
I to 2 inches in length, roughly sculptured or smoothish. Commonly