Cerion bimarginata Maynard, 1894

Original Description

"STROPHIA BIMARGINATA Novo.

Double-Margined Strophia.

Fig. 53, A, front view, B, side view, of type; C, young (enlarged),

D, double margin of another specimen.

DESCRIPTION.

            SP. CH. Size, medium. Shell, of medium thickness. Striations,

present. Whirls, ten. Examined 2,000 specimens. ­

            Form of shell, a pointed cylinder, the first, second and third

whirls being about equal in diameter, then the shell slopes gradually

to a rather blunt point, forming an angle of seventy degrees. The

striations are not very numerous, twenty-three to the first whirl;

they are rather regular, narrow, not quite as wide as the interspaces,

are inclined to be triangular, out are slightly rounded, smooth, but somewhat furrowed, are only partly arranged in lines and a little inclined from right to left.

            Aperture, quite small, in­clined to be rounded, open at the entrance, but contracted within. Lower tooth, promi­nent, double, with the second

smaller and placed on the left of the larger; the larger tooth is about .03 high, about as wide as high and not twice as long as high, the smaller one beside it is about half as large and is closely connected with the larger; both are somewhat elevated, are about central in position, and are placed back a little less than once the length of the larger from the central bar. The upper tooth is a mere tubercle, is elevated a little above the top of the lower tooth and

the space between the two is inclined to be filled up.

            Margin produced forward about as far as the diameter of the shell, it is thin, about as thick as the shell behind it, and is smoothly rounded, without being produced into any edge. Immediately behind

this first margin is a second margin with the edge roughened and

between this double margin the shell shows lines of growth, having, in

fact, formed the inner margin first, then, after adding more to the shell

formed a, new margin. The frontal bar is well developed and com­pletely interrupts the striations.

            Color of shell, externally, pure white, rather lustrous, internally,

pale brown, a little darker in shade well within the shell, and becoming white on the margin.

DIMENSIONS.

            Size of type, 1.07, by .45. Largest specimen, 1.20 by .55; smallest, .80 by .35. Greatest diameter, .55; smallest, .35. Longest specimen, 1.35; shortest, .80.

OBSERVATIONS.

            Individual variation is considerable and extends in several directions. The form is sometimes shorter and wider proportionally, much as in S. glans, and the margin is thinner and less inclined to be double, but is never beveled as it is, and differs from S. glans, in main­taining the small rounded aperture. Some specimens are also more cylindrical and smaller in diameter. The double tooth occurs only in perfectly adult specimens and is not always present then. This would also appear to be a character which seems strongest in indi­viduals which are placed in exposed situations, and thus now is rather of a secondary specific character, but as the species becomes more fixed, the double tooth will become more constant; it is present, how­ever, is about seventy-five per cent. of the specimens obtained on Green Key, the locality of the type, and in about 50 per cent., or a little less, of the specimens from Little Galden Key.

            The double margin of this species is a singular character, and one which I have not seen in any other species of Strophia. It is present in a more or less marked degree, in nearly all adult shells, but some­times the doubling is represented by lines of growth on a simple grooved and beveled edge; possibly, even in these cases, there will be another margin formed. In the example given in Fig. 52, D, there bas been two distinct margins formed, both beveled, grooved, and marked with lines of growth, a significant fact and one to which I will refer again.

            In this connection it is worthy of remark that I have a single specimen of S. glans in which two distinct margins have been formed, a. considerable distance apart, but I shall figure this specimen arid refer to it later.

            The doubling of the margin and of the teeth in this species appears to have sometimes caused reversion to some long past ancestral type, which was the common ancestor of an the sub-genera of the genus, for in this species we find that several teeth appear in young shells, at least I have one specimen, a very young shell, with five whirls only, in which five teeth appear, almost exactly as seen in the young specimen of S. pannosa and of S. festiva figured on Plate 11, Vol 1 of these Contributions. Compare these figures with Fig. 52, D, of the present volume, where I give a figure, twice enlarged, of a young S. bimarginata, taken on Green Key, in April, 1893. I have also another specimen, from the same place, that is older, which has,

besides the tooth in the usual place on the column, another on the upper portion of the walls of the aperture and two indications on the larva. Other older specimens have simply the one tooth as seen in all other members of the sub-genus Maynardia, but I should not be at all surprised to find this character appearing, at least occasionally, in very young specimens of other members of this sub-genus. As also showing a tendency to revert to ancestral types, I have two adult shells in which there is no indication of any teeth whatever. (See remarks regarding the form of sub-genus Eostrophia on page 163.)

            I have remarked that the type form of this species came from

Green Key, off Andros, but this species also occurs on Little Galden

Key, Middle Bight, Andros. But the shells from this latter-named

place here have caused me no little perplexity. They appear different

from those on Green Key, as they lie in mass in the boxes, side by

side, and I considered them different when I collected them, but when I

try to get at some exact character which will separate the shells from

the two islands I utterly fail to find any which will answer even to

represent a form. All can say, is, that, on the average, or taken in mass,

the shells from Little Galden Key are larger, coarser  and darker in

color, internally, and with an inclination to a creamy tint, externally.

            There are, however, two forms, one of which is also found on Little

Galden Key, and a well-marked sub-species, developed on Green Key.

            No.1 is a large, coarse form, with an inclination to assume a

bevelled margin and single tooth, with a darker interior. This form

also appears on Little Galden Key, but here is even larger with a

decidedly creamy color externally. On both keys the shell is found

in the thicket away from the beach.

            No.2 is a small form, the smallest dimensions given, with a thin

margin, and with a tendency toward a single tooth, although this

is sometimes doubled. The color is dicidedly bluish, and I have two

or three specimens which are considerably flecked. This form occurs

among the low grass of the rocks of Green Key.

HABITS AND DISTRIBUTIUN.

            All along the eastern coast of Andros stretches a barren reef which varies in distance from the island from one to five miles. This reef is continuous excepting where it is broken at the entrance of the bights or creeks that make into the land. Here and there, along this reef which divides the lagoon between it and the land from the deep water of the Tongue of Ocean rose little islets varying in size from the merest points of rocks, just awash by the waves at high water, to keys ten acres in extent which rose sometimes thirty or forty feet above high water. One of those large islands is caned Green Key. [Of keys of this name I know three; one near New Providence, one here on the coast of Andros and one (a large key) about thirty-five or forty miles south of it, but on the opposite or eastern side of the Tongue of Ocean.]

            This key contains about five acres of land. It is rather low, backed on the eastern or seaward side, by rough jagged rocks which are somewhat elevated, and, on the western side, along the lagoon, by lower, smoother reeks. Directly on the water the rocks are exposed, but back of them the key is covered with a dense growth of bushes. It was along the lagoon border that we found Strophia bimarginata.

            They were clinging in clusters of from five to twenty-five or thirty individuals to the grass and herbage along the margin of the bushes; but were seldom found on the bushes themselves, but deeper in among the bushes we found the larger form No.1. while form No. 2 was fouud on the lower herbage nearer the water.

            On Little Galden Key, Middle Bight, which I have described on page 29 of these Contributions, we found the shells in a similar situation, but never on either key on the eastern or seaward side. I also found a form, corresponding to No.1, in the woods at Little Galden Key."

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