DISCOVERY OF THE DISK OF ONYCHOCRINUS 475 



the Impinnata division of the Crinoidea FlexibiHa, dividing them 

 into two main family groups — Ichthyocrinidae and Taxocrinidae. 

 Some of Angehn's Silurian genera, as stated at that time, were 

 arranged with doubt. Since then I have received from Mr. G. Liljevall, 

 of the National Museum at Stockholm — who has at my request 

 made a careful examination of the Swedish material belonging to this 

 group — a set of line drawings of the principal specimens of Angelin's 

 genera, embracing all the types figured in the Iconographia Crin- 

 oideorum that can be found, and some other specimens not hereto- 

 fore figured. These drawings, made with the unrivaled skill and 

 care which characterize all of Liljevall's work, have made clear some 

 points hitherto obscure on account of the notorious inaccuracy of 

 Angelin's figures, and have also led to some unexpected results. In 

 view of the importance of some of these observations, and their 

 bearing upon the classification of this group, I have thought it advis- 

 able to publish a brief account of them now as a preliminary note 

 to the work upon it which I have in preparation. 



It was shown in the paper above cited that in the family Taxo- 

 crinidae there is a progressive variation in the position of the radianal, 

 similar to and somewhat parallel with that observed in the Inadunata, 

 viz. : from a primitive position directly under the right posterior 

 radial, to an oblique position under the lower left corner of that 

 radial in the Silurian, and finally to complete elimination in the 

 Carboniferous. It now appears that there is a similar variation and 

 succession in the position of the radianal among the Ichthyocrinidae. 

 Anisocrinus and Clidochirus prove to have a large radianal; in the 

 latter in a radial position directly underneath the right posterior 

 radial, as if it were the lower half of it, as in Temnocrinus; while 

 Lecanocrinus and Pycnosaccus have it smaller, and located obliquely 

 under the left part of the right posterior ray, as in Gnorimocrinus. 

 Homalocrinus also has the radianal in the primitive position, but it is 

 smaller than in the two first-named genera. In Mespilocrinus and 

 other carboniferous forms it has disappeared altogether. 



The greatest surprise, however, has been to find a somewhat 

 similar condition among forms which have been referred to the genus 

 Ichthyocrinus. The genus has been heretofore considered by every- 

 body as the simplest of all, having a perfectly symmetrical calyx. 



