DISCOVERY OF THE DISK OF ONYCHOCRINUS 499 



its entire range — except in certain peculiar transition cases to which I 

 will allude later — there is no suggestion, in the external form, of any- 

 such structure as an anal tube. 



Under this form, as well as the first, the modification reached the 

 phase of the entire disappearance of the anal plates, first in the 

 Silurian without disturbing the radianal, as in Ichthyocrinus, and 

 afterwards in the Carboniferous with complete elimination of that 

 plate, as in Metichthyocrinus. 



Looking at these well-marked examples of the two plans, one 

 cannot fail to be impressed with their complete distinctness as they 

 stand side by side in the Silurian (Plate VII, Figs. 16 and 18), and in 

 the Carboniferous (Plate VII, Figs. 17 and 20). They run a some- 

 what parallel course during the greater part of the paleontological 

 history of the group, but not to the end. The Sagenocrinus plan 

 ceases with the genus Forhesiocrinus in the upper part of the Keokuk 

 or Warsaw Limestone, while the Taxocrinus plan survives to the 

 last in the genera Taxocrinus and Onychocrinus. The latter is thus 

 the one character connected with the anal structures which survived 

 from the Lower Silurian to the extinction of the group in the higher 

 Carboniferous, and is continued to the present time in one genus of 

 the Flexibilia Pinnata. Only one specimen belonging to the Flexi- 

 bilia Impinnata is known from rocks later than the Kaskaskia, and 

 that is an Ichthyocrinoid from the Lower Coal Measures, of which 

 the calyx is unfortunately wanting. When its characters are made 

 known by future discoveries, I shall expect to find it with an anal 

 side of the Taxocrinus type. 



Notwithstanding the evident distinctness of the two plans, their 

 early divergence from a common origin which can with reasonable 

 probability be inferred, and their long duration as independent lines 

 of structure, it is nevertheless a curious fact that they also tend to 

 run together, in a sort of convergent evolution, toward the close of 

 their history. I can show most beautifully by actual specimens how 

 this has occurred in the Carboniferous between Forhesiocrinus and 

 Taxocrinus, not so much in the way of individual variations as in 

 the modification of species. We have a species of Forhesiocrinus in 

 the Keokuk Limestone with a firmly plated anal area, in which there 

 is a well-marked vertical series imbedded in the middle, but usually 



