DISCOVERY OF THE DISK OF ONYCHOCRINUS 493 



five rows of broad costal and scapular plates, four in each, the last 

 being cuneiform." As T. nohilis and T. macrodactylus have hitherto 

 been taken as the typical forms of the genus, and according to Phillips' 

 diagnosis they possess the brachial structure No. 2, it seems proper 

 to retain the name for the Carboniferous species and such Devonian 

 or Silurian ones as prove to have this structure. For those species 

 of Taxocrinus having the original structure of two primibrachs I 

 propose the genus Eutaxocrinus, including provisionally the Silurian 

 T. ohlongatus and T. rigens, which are probably one species, and 

 tend toward Dactylocrinus in the arm structure. 



In attempting to arrange the genera of the Flexibilia into families, 

 or other subgroups, we have to choose between several kinds of modi- 

 fication on which to base them. We know the life-history of one 

 genus of the Flexibilia Pinnata, viz., Antedon. Considering that to 

 represent, in a general way and to some extent, the phylogenetic 

 history of the group, we may assume that its ancestral form would 

 be something like the early larval stage of Antedon, with the addi- 

 tion of a radianal, of which no trace or suggestion has. yet been found 

 in the embryological researches on that genus, as I understand them. 

 This would give us a dicyclic Crinoid, with a radianal; an anal plate 

 between the posterior radials; two primibrachs, the second one 

 axillary and followed by arm branches; and the ventral side sur- 

 mounted by a closed pyramid of oral plates. I have attempted to 

 represent the dorsal side of such a hypothetical Crinoid by Fig. 9 

 on Plate II. The lines of modification from such a form on the dor- 

 sal side would be: 



a) In the radianal, by migration upward. 



h) In the anal system by (i) extension upward in vertical series 

 not connected with brachials, and replacement laterally by perisome; 



(2) increase upward and laterally in connection with the brachials; 



(3) elimination of the anal plate. 



c) In the brachial system by (i) increase in the number of primi- 

 brachs;'' (2) variation in the mode of branching of the higher brachials 

 or arms. 



d) In the interbrachial system, by growth and multiplication of 

 supplementary plates between the rays. 



