DISCOVERY OF THE DISK OF ONYCHOCRINUS 479 



noted) have a perfect pentamerous symmetry below the first bifurca- 

 tion, and thus have no radianal. 



Hence we have in this form an evolution from the Silurian to the 

 Carboniferous, just as is found from 



Sagenocrinus to Forbesiocrinus, 

 Gnorimocrinus to Taxocrinus, 

 Clidochirus to Mespilocrinus. 



The process here shown, substantially parallel with what takes 

 place in the Inadunata, is the elimination of the radianal in paleon- 

 tological time by rising, or being lifted upward, from beneath the 

 radial, analogous to the migration and disappearance of the anal 

 plate in the larval stages of Antedon, of which I shall have more to 

 say later on. If these views are correct, it follows that the Carbonif- 

 erous form should be separated generically from the Silurian. As 

 /. laevis, from the Niagara, is the genotype of Ichthyocrinus, it seems 

 proper to retain the genus for the Silurian species and any other that 

 may prove to have a radianal. I therefore propose for the Carbonif- 

 erous form, and all that may be without a radianal, the genus 

 M etichthyocrinus . 



As already intimated, Anisocrinus and Clidochirus, two of Angelin's 

 Silurian genera, also prove to have an unsymmetrical calyx, owing 

 to the presence of a radianal underneath the right posterior radial. 

 In Clidochirus it is directly under the ray, in the position of an infer- 

 radial (Plate VI, Fig. 7). This is shown in Angelin's figure of C. 

 pyrum.' The original to his Fig. 6, Plate XVII, which is a restora- 

 tion from an imperfect calyx, and incorrectly drawn as to the part 

 preserved, is a right anterior view of a weathered specimen of this 

 species ; the lower part of the calyx, showing the anal side, is figured 

 herewith (Plate VI, Fig. 8). There is also a third specimen in the 

 Stockholm collection which shows the characters perfectly (Plate VI, 

 Fig. 7). The difference between the two genera is that Anisocrinus 

 has the interbrachials all around, while Clidochirus has none except 

 at the anal side. The radianal of Anisocrinus is not quite so regular 

 in its position as in the other genera having this structure. In A. 

 interradiatus it is large, and well underneath the radial. Angelin's 

 figure of his only specimen, Plate XXII, Fig. 18, does not show any 



I Iconographia Crinoideoriim, Plate XXII, Fig. 23. 



