60 SMITHSONIAN INSTITUTION 



THE INTERRELATIONSHIPS OF THE FLEXIBIL1A 



In my paper of 1906, 1 which as then stated was a note preliminary to the 

 present work, I gave at some length the conclusions to which my investigations 

 had at that time led touching the interrelationships of the Flexibilia; these 

 conclusions remain for the most part substantially the same, and I will restate 

 them here with such additions and reinforcement as subsequent researches 

 have furnished. In attempting to arrange the genera of the Flexibilia in fami- 

 lies, or other subgroups, we have to choose for our differential characters 

 between several types of structural modification. At the time of my former 

 discussion the life history of only one genus of crinoids was known, viz., 

 Antedon. Considering that to represent, " in a general way and to some 

 extent," the phylogenetic history of the group, I assumed " 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 as 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 1 posterior 

 radials; two primibrachs, the second one axillary and followed by arm 

 branches ; and the ventral side surmounted by a pyramid of oral plates. I have 

 attempted to represent the dorsal side of such a hypothetical crinoid by figure 

 9 of Plate V" {op. cit., p. 493). I have reproduced the hypothetical figure 

 there given, both for use in this discussion for which it still serves very well, 2 

 and to show the remarkable way in which it has been confirmed by later dis- 

 coveries (PI. A, fig. 7) : 



First, by the independent researches of Mr. Clark, who in his paper of 



1912 3 on the anal plates of the Recent crinoids, and later in his monograph of 



1915, 4 says of it: 



In this connection it is most interesting to examine the figure published by Mr. Frank 

 Springer ' to show the probable primitive structure of the anal interradius and adjacent parts 

 of the calyx in the whole Flexibilia type, both fossil and recent. If we should carry backward 

 to its probable inception the course indicated by the migration of the radianal plate in the 

 young of the recent comatulids, we should arrive at a calyx structure identical with that 

 shown by Mr. Springer and deduced from the study of the fossil forms. From the study of 

 the recent types alone it might be argued that the figure should be slightly modified by the 

 reduplication of anal x in the shape of interradials in all the other interradial areas ; but from 

 the data acquired from the study of the six-rayed specimens, and the very evident modifica- 

 tion of all the recent types in the direction of a perfect, derived from an imperfect, radial 

 symmetry, it would seem that we would be justified in considering these four additional inter- 

 radials as a later development. 



'Journal of Geology, vol. 14, pp. 467 et seq. 



3 Although modeled upon Homalocrinus instead of the now preferable Protaxocrinus. 



3 Jour. Washington Acad. Sci., vol. 2, p. 313. 



4 Monograph of the Existing Crinoids, p. 339. 



6 Jour. Geology, 14, no. 6, 1906, pi. 5, fig. 9; explanation p. 493. 



