Page:Walcott Cambrian Geology and Paleontology II.djvu/90

 alimentary canal the diameter of the umbrella must have been about 12 cm. That the greater number of specimens were smaller is proven by the size of the spiral alimentary canal.

Occurrence.—All of the specimens found were in a layer of shale averaging two inches in thickness, and usually on the middle split of the layer. Trilobites of the genus Ptychoparia, several phyllopod crustaceans, and sponges occur in the same layer and often on the same surface with Eldonia ludwigi.

Observations.—To the zoologist acquainted with the Holothurioidea more questions will be raised by this remarkable fossil than I have answered in text or illustration. Perhaps the best way to present the case will be to relate my experience. When collecting in the summer of 1910, the specimens were noted as remains of a new and beautiful medusa. The following November the material was partially unpacked and examined, photographs made of several specimens, and at the Pittsburg meeting of the Geological Society of America, December 29, 1910, a brief description illustrated by lantern slides was given. The medusa was still appealed to, to explain the general structure, but only by considering the large, coiled, elongate body as a commensal annelid could the medusa view be retained. In March, with all the material unpacked and available, a preliminary study was made of the numerous associated annelids and the supposed commensal annelid, and the conclusion was reached that neither the medusa nor the commensal annelid view could be sustained. Dr. Austin H. Clark suggested that as the spiral alimentary canal was characteristic of the Echinodermata, it might be that this form was allied to the free swimming Pelagothuria. This led to a comparison with Pelagothuria natatrix Ludwig. I finally concluded that our new form was related to the holothurians, but that it was quite unlike Pelagothuria, the only described free swimming holothurian, and far more unlike the typical forms of the class. Except for the presence of the large spiral alimentary canal I should have returned to the medusa view at this point. There was no a priori reason why a holothurian should not have a medusa-like form, as noted by Dr. A. G. Mayer, but I found that the body of Pelagothuria was cylindrical; the disk an enlargement of the body at the base of the tentacles; and that the mouth opened at the dorsal surface, and the anus at the end