In his latest publication (1918, pp. 147-153, pls. 26-28, 33), Walcott has reviewed his earlier work on Calymene and Ceraurus, and presented a new restoration of the former. The coxopodites are now interpreted as being similar to those of Triarthrus and Neolenus, but the exopodites are still held to be spiral and the setiferous organs labelled as epipodites rather than exopodites.
Comparison of the Appendages of Calymene and Ceraurus with those of Triarthrus.
As one may see by reading the above quotations from Doctor Walcott's descriptions, he found certain branchial organs in Ceraurus and Calymene which have not been found in other trilobites but otherwise the essential features of the appendages of all are in agreement.
It is now necessary to inquire if the thin sections can not be interpreted on the basis of trilobites with the same organs as Triarthrus. The interpretation of the structures seen in these translucent slices is exceedingly difficult, and Doctor Walcott deserves the utmost praise for the acumen with which he drew his deductions. Even with the present knowledge of Triarthrus, Isotelus, and Neolenus as a guide, I do not think it is safe to speak dogmatically about what one sees in them.
Walcott has summarized his results in his restoration of the appendages of Calymene (1918, pl. 33). The coxopodite supports a slender six-jointed endopodite as in Triarthrus, dorsal to which is a short setiferous epipodite which differs from the exopodite of Triarthrus, in being less long, unsegmented, and in having shorter setæ. Arising from the same part of the coxopodite with this epipodite is the bifurcate spiral branchia which has not been seen in this form in other trilobites. The evidence on which the existence of this organ is postulated consists of a series of sections across the thorax, the best of them figured by Walcott in his plates 2 and 3 (1881) and plate 27 (1918).
The specimens sliced were all partially or quite enrolled, and in that position one would expect to find the appendages so displaced that it would be only rarely that a section would be cut, either by chance or design, in such a direction as to show any considerable part of any one appendage. This expectation has proved true in regard to the endopodites, the sections rarely showing more than two or three consecutive segments. Sections like those shown in figures 1 and 2 in plate 2 (1881) seem to be unique. On the other hand, there are numerous slices showing the so-called spiral branchiæ. They show for the most part as a succession of rectangular to kidney-shaped spots of clear calcite.[1] Usually these clear spots are isolated, not confluent, but in a small number of specimens, perhaps three or four, the spots are connected in such a way as to show a zig-zag band which suggests a spiral. Such an explanation is of course entirely reasonable, but it would be surprising if so slender a spiral should be cut in such a way as to exhibit the large series of successive turns shown in many of these thin sections. Continuous sections of such organs should be no more common than continuous sections of endopodites.
[1] In looking at Walcott's figures of 1881, it should be remembered that the dark portions of the figures are clear calcite in the specimens, while the light part is the more or less opaque matrix.
One of the arguments against the interpretation of these series of spots as sections across spiral arms is that of probabilities. It is known from flattened specimens that Neolenus, Kootenia, Ptychoparia, Triarthrus, and Cryptolithus all have a single type of exopodite, consisting of a simple setiferous shaft. All these genera have been examined in a way that permits no doubt about the structure, and no trace of spiral arms has been detected. On the other hand, Walcott found spiral arms in three unrelated genera, Calymene, Ceraurus, and Acidaspis, all of the trilobites in which he found exopodites by the method of sectioning. What are the probabilities that genera of three different families, studied by means of sections, should agree in having a type of exopodite different from that of the five genera about whose interpretation there can be no doubt?