DISCOVERY OF THE DISK OF ONYCHOCRINUS 505 



We may also be warranted in supposing, from the observed course 

 of development of the anal structures in the larva of Antedon as above 

 described, that the Taxocrinus plan represents the earliest and most 

 primitive form, and that the modifications of this in paleontological 

 time were those which tended toward the disappearance of the row 

 of anal plates, with its border of perisome, and the substitution of 

 regular calyx plates, suturally connected with the adjacent rays. 

 Hence the outcome of the struggle in paleozoic time was. the survival 

 of the original plan, and the suppression of the modified one. 



3. The third modification of the anal structures, which also runs 

 from the Silurian to the Carboniferous, is that in which the anal 

 plate has altogether disappeared, and it is merely a further extension 

 of one of the preceding. This occurred in the Silurian in the genus 

 Ichthyocrinus, while still retaining its primitive radianal, which it 

 did not get rid of until the Carboniferous, where, in the genus Metich- 

 thyocrinus, we have a Crinoid with perfect pentamerous symmetry, 

 so far as the external test shows. Two genera in the division with 

 divergent arms reach this stage also in the Carboniferous — Wachs- 

 muthicrinus, in which traces of bilateral symmetry can be seen in 

 the slightly larger size of the posterior basal, and Nipterocrinus, in 

 which the pentamerous symmetry seems to be perfect. This modi- 

 fication apparently did not much influence the history of the group. 



c) The first modification in the brachial system — i. e., in the num- 

 ber of primibrachs — has already been described and discussed as to 

 its details. There can be little question that the primitive form in 

 this respect was that with two primibrachs. It belongs to the Ante- 

 don larva, and it prevailed almost exclusively in the Silurian. The 

 few cases in which there is but one primibrach may be explained by 

 the syzygial union of two of the primitive brachials, just as happens 

 in some species of the living Antedon, without changing the funda- 

 mental plan. The addition of another primary brachial simultane- 

 ously in all of the rays, producing the 3TBr structure, cannot be 

 explained in any such way, and the occurrence of this form in the 

 Silurian is so limited and exceptional that it may scarcely be said to 

 have had a beginning before the Devonian. It is clearly the suc- 

 cessor of the first one, in the paleozoic. The two-IBr structure con- 



