PHYLOGENETIC RELATIONS 91 



the presence of pinnules, and of five infrabasals instead of three, differentiate 

 it from the known Flexibilia. On the other hand the brachial structure, the 

 form and arrangement of arms and pinnules, with ridges following the radial 

 lines and running into the arms, are essentially the characters of Glyptocrinus 

 or Reteocrinus. To the full description of this genus, and explanation of its 

 anomalous basal structure in which it differs from all other crinoids, as given 

 in my memoir of 1905, * I added in my Canadian memoir of 191 1 {op. cit., 

 p. 43, pi. 5, figs. 7-1 1 ) evidence proving that the mode of union of the calyx 

 plates is clearly that of the cystids, with the calycine pores and rhombs of that 

 group in abundance. The form is too symmetrically organized to remain with 

 the cystids, and it will not fit into any of the recognized orders of the crinoids. 

 Hence it must be treated simply as an intermediate form, evolved late from the 

 cystids, but whose crinoidal tendencies had not yet settled into a definite line of 

 development. 



Thus it seems that at this very early stage in the geological scale we have 

 several forms exhibiting variously intermingled characters of the larger divi- 

 sions of the crinoids, with some of the essential cystid structure more or less 

 impressed upon one of them; and that these represent relatively recent depar- 

 tures from the common ancestral type, tending in different degrees toward the 

 lines of evolution which produced the several orders of the crinoids. In Pro- 

 taxocrinus the Flexible characters were already well established; in Cupul- 

 ocrinus and Reteocrinus the tendency was toward the Inadunata and Camerata 

 respectively, while still complicated by other characters; while in Cleiocrimis 

 the strong survival of cystid characters prevented the establishment of a dis- 

 tinct evolutionary line in either of the crinoidal orders. 



The Flexibilia, like the Camerata, were limited strictly to the Paleozoic; 

 and if they left survivals these were so overshadowed by the great changes 

 which occurred in Mesozoic and Recent times, that they must be considered 

 as wholly subordinate, and not of sufficient weight to maintain this order as a 

 zoological concept. The calyx of Uintacrinus, with its profuse interbrachials 

 and flexible walls, seems at first glance remarkably similar to that of some 

 forms of the Flexibilia. But Uintacrinus is essentially a comatulid with re- 

 markable protean and convergent characters, as I suggested in 1901, 2 and as 

 has since been fully demonstrated by Clark. 3 The resemblance is superficial 

 rather than morphological, the very thin plates of Uintacrinus being- totally 

 opposed to the brick-like structure of the Flexibilia. 



The exposed mouth and ambulacra of the Flexibilia constitute, of course, 

 their most striking character which has come down to present time and which 



1 Cleiocrinus, Mem. Mus. Comp. Zoology, Harvard, vol. 25, no. 2, pp. 93-114. 

 * Uintacrinus, Mem. Mus. Comp. Zoology, Harvard, vol. 25, no. I, pp. 55 et seq. 



3 The Systematic Position of the Crinoid Genus Marsupites, Proc. U. S. National Museum, vol. 40, 

 191 1, pp. 649-654. 



