SOME STRUCTURAL FEATURES OF A FOSSIL EMBRYO 



CRINOIDi 



BY GEORGE II. HUDSON 

 (With I plate) 

 While recently engaged in the study of a specimen of the genus 

 Urasterella I found, associated with it, what appeared to be an 

 embryo crinoid. The structure of its arms and their branches, 

 however, revealed no definitely formed ossicles but in their place 

 there appeared to be a linear series of irregularly shaped tranverse 

 disks or semicircular wedges of developing stereom in an other- 

 wise fleshy extension of epidermal or associated tissues. This 

 structure was so like that of the spines and spinelets of Urasterella 

 medusa, as revealed through gum mountings under cover-glass and 

 photomicrographic stereograms (see plates i, 2, 4, and 5 in article 

 " On the Genus Urasterella with Description of a New Species " in 

 the annual report for 1915 of the Director of Science) that for a 

 long time I half suspected the specimen might be but a detached 

 fragment of this sea star and its crinoidlike appearance to be due 

 to an association of plates originally surrounding some body open- 

 ing which was protected by a cluster of unusually long spines and 

 connected with a tubular extension simulating a crinoidal column. 

 Both plates and spinelike processes were fairly commensurate in 

 size with those of the Urasterella next to which it lay (see upper 

 part of figure 2 in stereogram illustrating this paper) and, if a 

 crinoid, but two infrabasals, three basals, and two radials were 

 to be seen and these, in part, in vertical section. A portion of the 

 surface of the specimen had been removed by weathering but it 

 appeared as if the sharp eye of the collector (Dr Charles D. Wal- 

 cott) had noted the spinelike processes and cut along them with 

 a knife to expose them to their tips. How carefully this was 

 done is clearly revealed by figure 2. 



After encouragement given by certain paleontologists, to whom 

 I showed the stereograms, that I might trust my vision in this 

 matter, and after seeing, on reexamination, a vertical line of zigzag 

 sutures on the column, I finally exorcised the Urasterella ghost. 



^ Paper as read before the Paleontological Society, Albany, N, Y., Decem- 

 ber 28, 1916. 



[161] 



