50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



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 PtycJioparia, several phyllopod 

 crustaceans, and sponges occur in the same layer and often on the 

 same surface with Eldonia ludzvigi. 



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 par- 

 tially unpacked and examined, photographs made of several specimens, 

 and at the Pittsburg meeting of the Geological Society of America, 

 December 29, 19 10, a brief description illustrated by lantern slides 

 was given. The medusa was still appealed to, to explaim 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 tmpacked and available, a preliminary 

 study was made of the numerous associated annelids and the sup- 

 posed 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 pres- 

 ence of the large spiral alimentary canal I should have returned to 

 the medusa view at this point. There w^as 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 



"^Mem. Mils. Comp. Zool., Vol. 17, 1894. No. 3, p. 114. 



^ Medusfe of the World ; Publication No. 109, Carnegie Institution of Wash- 

 ington, 1910, Vol. 3, p. 499. 



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