Cerion evolva Maynard, 1894
"STROPHIA PILSBRYI EVOLVA Novo.
Pilsbry's Mottled Strophia.
FIG. 57, A, front view, B, margin, of type.
SP. CH. Size, rather small Shell, rather heavy. Striations, present. Whirls, ten. Examined seventy specimens.
Form of shell, a pointed cylinder, with the first and second whirls equal in diameter, the third is but little smaller, then the shell slopes to a point, forming an angle of sixty two degrees. First whirl, short, equal to the next four smaller. The striations are not numerous, twenty-two w the first whirl, are rather regular, but somewhat broken, narrow, not as wide
as the interspaces between them, smoothly rounded, polished, but some of them are slightly furrowed, and the interspaces are slightly ridged with lines of growth.
Aperture, rather small, arched, about as high as wide, and is slightly wider within than at the entrance. Lower tooth, not very prominent, .03 high, wider than high, and a little more than twice as long as high, it is not elevated, but the floor of the entrance slopes away from it, and the tooth is projected forward nearly to the frontal bar. Its position is about central. Upper tooth is a mere tubercle.
Margin produced forward about as far as the diameter of the shell, and is considerably inclined toward the right, quite beyond the diameter of the shell, it is thick, about .07, beveled, grooved, and very faintly marked with lines of growth. The frontal bar is quite well developed, protruding slightly beyond the striations, and is very obliquely inclined to the right.
Color of shell, externally, ashy yellow, mottled and blotched with ashy brown, the mottlings assuming a transverse direction, somewhat as in Strophia mumia; internally, dark brown, which color extends quite to the margin, leaving the tooth, however, pale, and the margin nearly white.
Size of type, 1.12, by, .43. Largest specimen, 1.17 by ,43; smallest, .90 by .40. Greatest diameter, .43; smallest, .40. Longest specimen, 1.17; shortest, .85.
Individual variation is great, and the most noticeable is toward producing a white form, which is clearly a reversion toward typical S. pilsbryi, in fact, this form grades directly into the parent stock.
Another variation, which may be considered as progressive, is toward producing a shorter, thicker form, with nine whirls and with numerous striations, thirty to the first whirl, they are regular, and not wider than the interspaces, and the first whirl is as long as the first five, while the aperture is inclined to be rounded, and more in the centre of the shell than in the type. The color is also dark, nearly uniform ashy brown, but with central tooth as in the type. I notice this form particularly as I believe that it will be found somewhere, at some time, as an established species.
Directly opposite this is a larger form, more cylindrical, with the first three whirls equal in diameter, and with the transverse mottlings more pronounced. This form seems to mark the direction in which the sub-species is trending to establish a full-ranked species. Occasionally a specimen will show a double margin.
As will be seen, the characters which mark this fine sub-species are, first, and most important, the elongated central tooth projected well forward, toward the frontal bar, a remarkable character and one which I have never seen so prominent in any other species of this genus; second, the form of the margin, with its oblique bar; thirdly, the peculiar mottlings, recalling those of S. mumia. Besides, we have the usual characters which almost always accompany the evolution of a dark, mottled form of shell from a white form, namely, the thickening of the shell and contraction of the margin. All of these characters in combination, together with the fact that this form has a location of its own, go to make up good and sufficient specific characters and I would so consider them, but there is yet too great a number of specimens which must be considered as intermediate.
HABITS AND DISTRIBUTION.
This strongly marked sub-species occurs on the bushes of the western and larger portion of the Goat Key, (E, in the chart, fig. 55,) but along; its southern border; I have marked its range with a series of *s. The form is not abundant in numbers and rather scattering."