DISCOVERY OF THE DISK OF ONYCHOCRINUS 469 



at that locality is poor. At Burlington specimens of this genus have 

 not been found in a matrix soft enough to permit the exposure of 

 such delicate structures. At Le Grand, where the Taxocrinus disk 

 was found, Onychocrinus does not occur — not having been developed 

 in America, so far as yet known, earlier than the Upper Burlington 

 Limestone. 



The only known material which seemed to offer a possibility of 

 discovering the tegmen of this genus was that from Indian Creek, 

 Montgomery County, Ind., where Onychocrinus ulrichi was obtained 

 in considerable numbers; many of them with the rays completely 

 spread out horizontally, and imbedded in a fine-grained, homogeneous 

 matrix, admitting of minute cleaning. In collecting, these were usually 

 found with the dorsal side exposed and the ventral side embedded, 

 and when the collection was first made from that locality, several 

 such specimens were cleaned without yielding the desired result. 

 Remnants of the disk were observed in some, broken and lying in 

 confusion in the bottom of the cavity, and showing but Httle of the 

 structure. It was clear that the disk was of such extreme flexibihty 

 that it had been forced by the pressure of the calcareous matrix down 

 into the dorsal cavity, and was generally broken and displaced. 

 Recently I undertook the search afresh, and by very delicate manipu- 

 lation had over a dozen of the best specimens with outspread rays 

 completely freed from the matrix on the ventral side — a very tedious 

 and laborious operation, and sometimes sadly destructive of fine 

 specimens. Some w^ere dissected by the removal of the calyx plates, 

 in hope of gaining access to the under side of the disk. 



These efforts were finally rewarded by uncovering, in several 

 specimens, the tegmen in such position as to give a complete elucida- 

 tion of the structure. The exceptional condition of the specimens, 

 and the exceedingly fine and homogeneous character of the material 

 in which they were imbedded, were all that rendered the discoverv 

 of these details possible. With the light afforded by them several 

 others in which the structures were only partially preserved — though 

 much displaced — became easily understood. The construction of 

 this disk is upon the same general plan as that of Taxocrinus, but 

 exhibits a very interesting modification of it. It is gratifying to find, 

 in this new discovery upon an additional genus, a full confirmation 



