i 3 2 SCIENCE PROGRESS 



became similarly anchylosed in the solid, dome-like tegmen. 

 In this manner there arose massive, plated forms like Eucalypto- 

 crinus (with an anal tube so large that the arms lie in grooves 

 of it), Batocrinus, and Actinocrinus among the Monocyclica and 

 Rhodocrinus and Rcteocrinus among the Dicyclica. 



In the Silurian the monocyclic section gave rise to another 

 offshoot — the Adunata — in which a somewhat similar rigidity 

 was secured by the tegmen growing into a solid mass and 

 the arms becoming biserial but the calyx was not affected to 

 so great an extent, e.g. in such forms as Platycrinus, which 

 attained its maximum development in the Carboniferous and 

 Hexacrinus in the Devonian. 



All these forms in which the armour was increased and its 

 elements anchylosed together so as to secure greater rigidity 

 of structure, failed to survive the great alterations of environ- 

 ment which took place at the close of the Palaeozoic. In the 

 Mesozoic era the only forms of the Monocyclic Crinoids which 

 could persist were simpler and more generalised forms, such 

 as Plicatocrinus, from which the Upper Jurassic free-swimming, 

 stemless Saccocoma is considered by Jaekel 1 to have evolved. 

 The adaptation of the latter to a pelagic existence is exempli- 

 fied by the thin and flexible character of its calyx and by the 

 manner in which the skeletal tissue in general has lost its 

 dense nature, showing a loose and reticulated structure. The 

 arms also are relatively slender, with only scattered pinnules 

 and possessed the power of rolling themselves up in a marked 

 degree. Even in Hyocrinus, which is the modern, living repre- 

 sentative of the Monocyclic Crinoids, the plates of the calyx 

 are comparatively thin and the basals are reduced to three 

 in number. 



In the Inadunate section of the Dicyclic Crinoids, the 

 Cyathocrinoidea (Cyathocrimts, Cupressocrinus, etc.) were the 

 most specialised ; hence the last survivors of this group died 

 out altogether in the Permian. On the other hand, the Distincta 

 or Dendrocrinoidea possessed more generalised characteristics 

 and were therefore able on occasions to give rise to more 

 flexible forms — the Flexibilia — which in Palaeozoic times pos- 

 sessed simple arms without pinnae (e.g. Ichtkyocrinus, Taxocrinus, 

 Dactylocrinus among fixed forms and Edriocrinus, which was 

 free-swimming in the adult condition). Again, in Mesozoic 



1 Zeitsch. deutsch. geol. Ges., xliv. 1892. 



