Strophia robusta Maynard, 1894
"STROPHIA CINEREA ROBUSTA Novo.
Fig. 36, A, front view; B, side view of type.
SP. CH. Size large. Form robust. Striations, present. Whirls, nine. Examined, seventy-five specimens.
Form of shell, conical; the Upper whirl being the largest, and each successive whirl smaller in regular proportion to the fourth, where the shell slopes abruptly to a blunt point, forming an angle of sixty-eight degrees. The striations are not numerous, twenty-three to the first whirl, are rather prominent, quite regular, but not arranged in lines, nearly straight on the first four whirls, but are slightly inclined from right to left on the lower whirls; they are not furrowed but are smoothly rounded, and are about as wide as the interspaces between them.
Aperture, small and contracted at the entrance, measuring a little more within than at the margin. Lower tooth, quite prominent, about .04 high, less than twice as long as high; it is slightly elevated. The upper tooth is about as high as the lower, and extends backward around the column.
Margin, not produced forward as far as the diameter of the shell, is slightly inclined to the right, but not beyond the diameter of the shell, but the upper whirl bulges out from it on the left, about half the width of the aperture. The margin is not thickened, and the only lateral projection from it is on the left, and that is slight. The frontal bar is thin, but interrupts the striations.
Color of the shell, externally, ashy brown, with the striations creamy white. Internally, dark brown, creamy white on the edge of margin and on teeth.
Size of type, 1.12 by .55. Largest specimen, 1.37 by .50; smallest, .97 by .30. Greatest diameter, .55; smallest, 30. Largest specimen, 1.25; shortest, .97.
Individual variation is considerable, but is mainly toward a cylindrical form with a thickened margin through which this subspecies intergrades with typical S. cinerea. but through a small percentage of specimens, some of which have ten whirls. The margin of some specimens is also thickened. A decided form is as follows:
No.1, smaller, 1.00 by .50. Whirls, still nine, but with the striations much more numerous, 26 to the first whirl. The upper tooth is not as prominent, and the color of the shell is paler.
The Robust Strophia may be distinguished at once in the type form from S cinerea by the nine instead of ten whirls, but there is, as stated, some gradations toward the parent stock.
HABITS AND DISTRIBUTION.
The Robust Strophia occurs on the north side of Hog Island among some dwarf palms that grow directly back of the beach ridge from a sandy soil. They are found clinging to the palm stems or leaves. The situation in which they are found is rather sheltered and the thickened shell has doubtlessly been evolved as a protection against the dryness and heat of the sand among which they live. The lack of any extended margin in both this and the parent stock would seem to indicate that the animal did not protrude itself as far from the shell as is usual in some other allied species.
The small form, No.1, occured beneath palm leaves in groups of five or six, but neither form were abundant.
One of my chief reasons for considering this form as a sub-species only, is that I found an occasional specimen of typical S. cinerea, scattered along a path which led directly from the beach to Middle Bay, on the south side of the island, which is the home of the Cinerous Strophia. These shells are doubtlessly transported accidentally by persons who pass along the path, and thus as the colony of S. robusta
receives an occasional recruit from the parent stock, direct intergrades occasionally occur."