Glyptocrinus decadactylus

“The column is round, composed of alternately thicker and thinner plates, the former projecting, and perforated with a pentagonal canal. Calyx obconoidal, interradial and intersecondary radialareas flattened in the lower half and more depressed above, with intertertiary areas deeply sunken, and strong radial ridges. There are five thick, sculptured, pentagonal, basal plates, about as wide as high, and about one fourth as large as the first primary radials. The lower part of the plates have a flange, which united forms a ring a little larger than the top of the column. The primary radials are 3 by 5, the first heptagonal, second hexagonal, and third heptagonal. The secondary radials are 2 by 10. The tertiary radials are 5 to S by 20, above which the arms are free. The regular interradial areas have one plate resting upon the primary radials, two in the second range, three in the third, two or three in the fourth, and above these fifteen or twenty small plates in each depressed intertertiary area. Intersecondary radial areas have one rather large plates in each axil, and a dozen or more smaller ones filling the depression between the tertiaries. Intertertiary areas have in like manner one plate in each axil, and several smaller ones above. Azygous area has one plate resting upon the primary radials, three in the second
range, three in the third, three or four in the fourth, four in the fifth, and above these thirty or forty small plates filling the depression between the tertiaries. A very strong ridge arises at the center of the first azygous plate, and extends straight up the center of the azygous area to the fourth range of plates, and continuing, gradually diminishing in size, seems to disappear at about the eighth plate, or between the second or third tertiaries. Arms twenty, long, rounded on the outer side and furrowed on the inner, and composed of cuneiform plates, each of which supports at its larger end a pinnule composed of joints three or four times as long as wide. The pinnules begin on the fourth or fifth tertiary, so that three or four occur on each side below the top of the vault. They are directed upward on each side of the extension of the vault, over the ambulacral furrow. I have entertained very erroneous opinions respecting it. I am glad, therefore, to be able, through the assistance of specimens from the magnificent collection of Dr. R. M. Byrnes, to more particularly and more fully describe it.It is somewhat convex in the central part, and undulates toward each intertertiary area. It is composed of numerous polygonal plates. Those in the central part are the larger ones, and each of these bears a central tubercle, which is sometimes prolonged so as to be designated a spine. Toward the margin, or rather following the undulations toward the intertertiary areas, the plates are smaller and possessed of slight convexity. They unite in the depressions in the intertertiary areas with the plates of the calyx, or rather the interprimary radials graduate through the intersecondaries and intertertiaries to the plates of the vault without any line of separation. The plates become smaller as they approach the inner face of the arms, over the swelling undulations of the vault, and continuing to decrease in size, forma somewhat granular, continuous integument, that covers the ambulacral furrows. This continuation of the vault up the inner side of the arms has been observed for the distance of an inch above the vault, and no doubt extended as far as the arm furrow itself. The pinnules do not cover the ambulacral grooves. Notwithstanding the magnificent pinnules upon the sides of the arms, in this genus, the ambulacral furrows are covered with small plates between the rows of pinnules, and the pinnules have nothing to do with covering the furrows. The reason emphasis is placed upon the fact is, Wachsmuth and Springer have doubted and even denied it. They say in their ” Revision of the Palseocrinoidea,” p. 25 : “It is important to note, that in those genera in which the ambulacral groove is thus covered, no regular pinnules have ever been observed, and, moreover, the construction is such that no additional pinnules could have existed ; while on the other hand no covering has ever been discovered in forms with true pinnulse.” And, finally, they come to the conclusion that the plates coveringthe ambulacral groove were homologous with the pinnulae, or, as they say, ” in fact rudimentary pinnules.” It is not easy to understand how pinnules should, in any case, act as a covering to an ambulacral groove, and as I have never seen a specimen which was calculated to make any such impression on my mind, I am unable to realize how those authors arrived at such a conclusion at least with regard to this genus, I beg leave to call the attention of the reader to the illustrations of this species on Plate XL, as well as to what is here said. The excurrent opening is situated subcentrally upon the upper face of the vault. It appears as a subcircular, depressed conical elevation, composed of plates, imbricating toward the center, while a few of the surrounding plates have rather long spines, inclined toward the central part of the orifice. One specimen showing the lower side of the vault, indicates the extension of a sack like form into the cavity below this excurrent opening, and it may be, that there was such connection with the ambulacral furrows, that this opening needed such support, but as I am unable to distinguish the plates, in this apparent extension, I do not assert the existence of it as part of the structure of the crinoid. Figure 1c, on Plate XL, is magnified six diameters, the upper part of the illustration reaches the larger plates of the central part of the vault, while the side spinous plates are those following the waves toward the arm furrows. It being definitely settled that the pinnules do not cover the arm furrows, the question very naturally suggests itself as to whether or not they had any connection with them. If they were perforated they had communication with the arm furrows, and if they were not perforated, they did not. Prepared microscopic sections would, probably, settle this question, and I will let it remain unanswered until someone prepares such sections. The surface of the calyx, in this species, is ornamented with strong radiating ridges, from the center of each plate, in the lower half, so as to form triangular depressed areas, which become less conspicuous in the middle part, and fade away in the intertertiary areas.
Found in the middle part of the Hudson River Group.”

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