468 FRANK SPRINGER 



nor any part of the perisome. Messrs. Lyon and Casseday, in their 

 description of Onychocrinus exsculptus,^ gave a partial description 

 of the vault, which they thought to be similar to that of the Asteroidea. 

 They observed, extending toward the arms, "five rays composed of 

 two rows of large, granulose pieces, one row alternating with the 

 other," and in the fields bordered by these five rays some interstitial 

 pieces which were "very small, granulose, and arranged without any 

 apparent order." They were unable, from the fragmentary condi- 

 tion of their specimens, to obtain any information as to the central 

 portion. 



As the genus Onychocrinus represents the most specialized form 

 of the group — one of the latest developed and latest surviving — it 

 has seemed a matter of special interest to ascertain the exact struc- 

 ture of its ventral side. Its calyx exhibits, in a striking degree, the 

 two characteristics of the group — massiveness and flexibility. Except 

 at the very base, where they are rather thin, the plates are thick and 

 heavy, and are closely laid together, like bricks in a pavement. The 

 interbrachial system connecting the rays dorsally is composed of 

 strong and thick plates, while the brachials are of great depth, and 

 have a very shallow ambulacral groove. Nevertheless, the plates of 

 this powerfully built calyx must have possessed an immense amount 

 of mobility among themselves, for we find it preserved in a great 

 variety of positions. In some specimens the rays are folded together, 

 and in others they are spread out horizontally. These changes of 

 position, while they involve all the brachial and interbrachial plates, 

 did not cause any marked opening or gaping of the sutures; but in 

 some singularly perfect manner the plates seem to have been capable 

 of sliding upon each other by their apposed surfaces, and thus accom- 

 modating themselves to any degree of flexure. 



The tegmen of such a calyx must necessarily have been of an 

 extremely pliant nature. But all attempts to obtain it, in sufficiently 

 perfect condition to disclose the nature of the mouth and oral struc- 

 tures generally, have hitherto proved futile. At Crawfordsville, Ind., 

 where two species have been found in large numbers, the specimens 

 almost always have the rays so closely folded together that the teg- 

 men cannot be got at, and, besides, the preservation of fine details 



I American Journal of Science, (2), Vol. XXIX, p. 79. 



