142 Cincinnati Society of Natural History, 



by means of pores, passing from one to the other. This series of plates 

 is connected b}^ a canal or central opening throughout its length, as is 

 also the larger series. 



The radiate structure consists of pores or passage ways from the 

 inner whorls to the outer ones, and commencing at the center of the 

 disk, which, in some of the casts, resembles the spokes of a wheel. A 

 passage way, through each of the finger-like processes, connects each 

 plate of the inner larger whorl, with each plate of the next outer larger 

 whorl, b}^ reason of a bifurcation in the passage way to unite with the 

 two plates, from which the finger-like processes arise. Thus doubling 

 the radiations with each succeeding whorl. The passages from the 

 smaller whorls to the larger ones are yqyj numerous, and taken in 

 connection with the other passages make the circular and radiate 

 structure exceedingly complicated. 



All of the specimens are casts, collected b}' W. 0. Egan, Esq., in 

 the magnesian limestone of the Niagara Group, at Bridgeport, near 

 Chicago, Illinois. From the numerous specimens which he collected, 

 he has ver^^ kindl}' presented some to. the author, and others to the 

 Cincinnati Society of Natural History. 



I regard the affinities of this genus with Cyclocystoides^ rather than 

 with any other genus known to me. The order Cystoidea has become, 

 however, the receptacle for too many remotely connected genera, and 

 I am inclined to think, that this genus belongs to an undefined order of 

 the Echinodermata. It is certainly not a crinoid, nor is it the finger of 

 one. 



In 1852, Prof. James Hall proposed the generic name Myelodactylus, 

 to distinguish what he supposed to be the arms or fingers of an otherwise 

 unknown crinoid, the remarkable feature of which, he said, is the foramen 

 or medullary canal penetrating the column of joints. He described 

 3fy elodactyliis convolutus 2i^ '-'• Q,ov[i'^osQdi of a. single series of thin joints, 

 which are slightly nodulose or tubercular on the back* ; ends of the joints 

 somewhat semicircular or crescent-form, with the extremities truncated ; 

 tentacula composed of numerous rounded or slightly nodulose joints, 

 which are attached to the truncated extremities of the finger-joint by 

 a tendon inserted into a perforation in the joint ; fingers usually in- 

 curved or convolute ; plates penetrated vertically b}^ an oblong quad- 

 rangular canal, through which probabl}^ passed a strong tendon con- 

 necting the whole together ; surface of each plate marked by a pen- 

 tagonal depression, within which are elevated ridges, the whole in- 

 tended for the strong attachment of muscular fibres connecting the 

 plates ; outer edge of the plate, upon the back, marked by two or 



