The Kansas University science bulletin, Vol. I, No. 6, September 1902

by Unknown

With Plate VI.

Owing to the systematic importance and the rarity of good material of the brachial framework of the brachiopods, any light on the extent of individual variation of these parts is of considerable importance. In the spring of 1899 the University of Kansas received from Prof. C. N. Gould a set of ten specimens of Seminula argentia (Shepard) Hall that show the position of the spires. These specimens were all from the same horizon in the Lower Permian. The University also had another specimen showing these characters, as did a specimen in the writer’s collection. Recently the study of these specimens was taken up and some remarkable results developed.

Both valves of this species are quite convex and not infrequently as broad as long. The older specimens are quite ventricose. However, the shell is subject to a considerable variation in form. Four of the twelve specimens studied were somewhat compressed, but it so happens that three of these approach the normal type very closely, while the fourth does not vary from it greatly. Those showing greatest variation have not been subject to any visible external deformation. The specimens under discussion are of about the average size and form.

The normal position of the spire is with its apex pointing to the side, near the line where the valves meet, at, or a little in front of, the middle of the shell, which is also the widest point. In the central part of the shell cavity the edges of the spires nearly meet. In front they flare apart, leaving a large, subcircular opening. For convenience, in this paper, this opening will be spoken of as the frontal aperture of the spiralia. It will also be necessary to orient the specimens so that definite positions may be referred to. For this purpose we will consider the specimens as front toward (back away from) the observer, with the brachial valve uppermost.

A specimen from the Topeka limestone, Upper Coal Measures, at Topeka, Kan., shows the spires with the apex of one of them pointing almost directly forward toward the anterior end of the shell, turned through a horizontal angle of about ninety degrees from its normal position, while, as nearly as may be determined from the specimen as cut, the apex of the other one is directed toward the median line of the pedicle valve just in front of the hinge. This specimen was selected to be ground, because it was a good specimen, of normal form. Another specimen (No. 3), from the Permian of Cowley county, Kansas, has the apices of the spires turned at an angle of about forty-five degrees or more in a vertical direction, causing the point of the spire to be located near the middle of the right side of the brachial valve, while the other points to the opposite of the pedicle valve. The spire is quite flattened, approaching disk shape, with the apex quite obtuse and the frontal aperture very narrow and almost slit-like (this may be partially due to a very slight compression, but the compression, if present, is so slight as to modify it very little), owing to the position of the spires in the shell, which prevents their flaring much at the front. The shell is not an old one, and, for this reason, is somewhat less ventricose than many adults. The young specimens are much flatter than the old ones in this species.

The spiralia of No. 10 are turned in a similar manner, but through a much smaller angle. The frontal aperture is typical, as is also the general form of the shell. The spiralia are conical, and the tips probably acute. In No. 6 the position of the spiralia, their form and that of the aperture are normal. Nos. 6 and 9 show spines on the spires. The spines are closely set, thick, blunt, and nearly twice as long as broad. In fact, some appear to be nearly as large at the tips as at the base. In No. 9 the spires are normal, except that, instead of being flaring conical, they are more in the shape of a folded shield shape with acute apex. In other words, the frontal aperture is produced by the dorso-ventral compression of the entire cone rather than the flaring of the frontal portion. The apex of the left spire is bent somewhat downward. The position of the spiralia in No. 7 is about normal, the tips obtuse, the spires almost perfectly depressed conical, frontal aperture only slightly wider than the space farther back on the side next the brachial valve. No. 5 has apparently been compressed laterally, through this compression did not affect the positions of the spiralia, for they are normal. They appear to have been of the typical form in every respect. In No. 8 the spires seem to have been turned through a slight vertical angle, though the incompleteness of the specimen prevents a close study of position. The form was apparently normal, except that the lower edges may have been somewhat pressed inward. Nos. 1 and 4 are about normal throughout. No. 2 is normal as far as can be seen, except that the apices are turned through a small vertical angle. The anterior portion of the pedicle valve is crushed in. No. 12 is normal throughout.

It is unfortunate that none of the specimens show the crural attachment of the spiralia. Such structures must certainly vary in order to support the spires in their various positions.

The above facts would seem to indicate the following conclusions: First, in those spire-bearing brachiopods in which the form of the shell does not govern the position of the spires, the Athyridæ in particular, the spires may be subject to a considerable variation in both position and form. Second, that the crural supports are probably so modified as to accommodate the spires in their various positions.

In the light of the foregoing, it will be seen that in the future it will be necessary to study the structure of several specimens before using small variations of internal structure in these shells as bases for group divisions.

One might expect greater individual variation among the Flint Hills specimens, owing to the physical changes that were taking place during the close of the Carboniferous and the early Permian; changes which soon caused this species, together with many others, to become extinct. However, this cannot be said of the most striking case (No. 11) from the Topeka limestone. It would seem that such variations as are found in Nos. 3 and 11 would be decidedly detrimental to the well-being of the animal, though the latter of the two seems to have been a vigorous individual.