The adolescent shell is beautifully spirally wavy-striate; in the adult the striae are near the ends only, or wholly absent. It varies from fine deep plum-color or sulphur-yellow to white. The yellow ones are found on the yellow, and the purple ones on the purple Gorgonias or sea-fans, respectively. The larval shell has four whorls, while one or two more turns may be made by the adolescent shell (about 3.5 mm. long) before the former is entirely hidden. The apex of the nucleus is blunt and of a reddish color, while the surface of the rest is paler and covered with sharp lirae, close set and parallel with the lines of growth. The sutures are evident but not pronounced. The adolescent shell, exclusive of that which is truly larval, is of a whitish color in the specimen examined, and has only fine spiral Cylichna-like striation. It has a distinct columella, sharply truncate in front, and a wide canal, while at the apex the shell is wound obliquely back over the nucleus, hiding about one half of it, and is produced in a short canal beyond the extreme tip. There appears to be a little over a whorl and a half subsequent to the original larval shell, and it would require just about one whorl more wholly to enclose it. The larval shell would seem to have been about a millimeter long, or perhaps somewhat less, before the adolescent part was begun.

One-tooth Simnia
Distribution: Virginia to east and west coasts of Florida, Texas; Brazil. Size: 12 to 19 mm
Description: Color orangish-brown with outer lip and plication white; shape elongate, narrow, and acuminate at both ends; sculpture smooth with irregular, spiral lines anteriorly and posteriorly; aperture almost as long as shell; posterior end pinched in with a single spiral plication; outer lip round and thick, anomphalous. Habitat: Attached to the gorgonian coral Eugorgia virgulata. Depth range 1 to 116 m (3 to 380 ft).
Remarks: Occasionally found in beach drift from Port Aransas and southward. Similar to S. marferula; however, S. marferula is generally broader and larger and has fine spiral striations throughout the dorsal surface. S. uniplicata has spiral striae posteriorly and anteriorly. See Cate (1973); Andrews (1977).

From the Antilles northward to within 20 miles twenty miles off Cape Hatteras, North Carolina.