MORPHOLOGY 57 



half of the right posterior radial, not touching the right anterior, between two 

 basals and meeting the infrabasals {Thenar ocrinus) ; (3) obliquely at the lower 

 left of the right posterior radial, resting upon the right shoulder of the pos- 

 terior basal, and not touching the right anterior, — lower oblique position 

 (Botry ocrinus, Poteriocrinus, etc.) ; (4) at the upper left of the right posterior 

 radial, pushed upward on the right shoulder of the elongated posterior basal,— 

 upper oblique position; (5) entirely absent, or perhaps assuming the function 

 of a symmetrically placed anal plate (Cyatliocrinus). To these might be added 

 for better comparison the condition in which all anal plates are eliminated from 

 the calyx (Erisocrinus and Encrinus). I have illustrated most of these posi- 

 tions by reference to Inadunate genera; the plate does not occur in the 

 Camerata. 



There is also the symmetrically situated plate found in all three orders of 

 the crinoids, designated as " Anal x," which occurs concurrently with the 

 radianal in many of the genera, and whose exact function or relation is not 

 clearly settled. It has been variously assumed to represent either the first inter- 

 brachial of the regular areas; the first plate of the anal tube; or an anal element 

 suddenly introduced. Without attempting to choose between these hypotheses, 

 I shall simply treat it, as I have done heretofore, as a distinct element of the 

 anal structures which may possibly at one stage be replaced by the radianal. 



Now in the Flexibilia the radianal passes through precisely the same modi- 

 fications as above described, in the same chronological succession; and we find 

 that there is in this group a series of generic forms based upon the migration 

 and disappearance of the radianal parallel to that occurring in the Inadunata, 

 and also for the most part parallel as between its own two larger divisions. 

 For we have Teumocrinus, Protaxocrinus (Ordovician) and I chthy ocrinus 

 (Silurian), like Cupulocrinus, with RA directly under the right posterior ray; 

 Sagenocrinns and Homalocrinus (Silurian), like Thenarocrinus, with RA 

 shifted somewhat to the left and located within the basal ring; Lecanocrinus 

 and Gnorimocrinus (Silurian), like Botry ocrinus, with RA shifted further to 

 the left and upward to an oblique position under the left lower corner of the 

 right posterior radial; Lithocrinus, Forbesiocrinus and Taxocrinus (Silurian 

 to Carboniferous), with RA further elevated and resting in a notch formed 

 by the sloping upper corners of the posterior basal and right posterior radial; 

 Hormocrinus like Cyatliocrinus (Silurian to Carboniferous), with RA not 

 identified, there being only symmetrical anal plates above the posterior basal; 

 Nipter ocrinus and Metichthy ocrinus (Carboniferous) like Encrinus, with all 

 anal plates eliminated from the calyx. 



Thus there was in both these orders an evolution from the Ordovician 

 and Silurian to the Carboniferous, in which the radianal passed from the 



