918 



REPORT — 1898. 



symmetry of subvective aud ambulacral systems induced completely correlated 

 symmetry in the theca. Tbe Diplopoiita, howevei', tend gradually aud clearly in 

 this direction, and lead imperceptibly to the Blastoidea. Either the Blastoidea, 

 as hitherto understood, must be placed with Oystidea, or their limits must be 

 enlarged to include such Diploporita as base this definite correlation and plates 

 that can be called ' radials.' The latter course enables us to define the Cystidea witli 

 greater precision, and is therefore adopted. 



The Blastoidea, therefore, have to be divided into two grades — Protoblastoidea 

 and Eublastoidea. The classification of the latter into Regulares and Irregularea 

 is no statement of genetic affinity. Moreover, Etheridgo and Carpenter's families 

 of the Regulares are based almost entirely on the relations of the hydrospire- 

 canals to the deltoids, relations which may vary considerably even in an indi- 

 vidual. Renewed study of the relations of the liydrospires to the ambulacra, of 

 persistent peculiarities of ornament, and the affinities of individual genera, suggests 

 that there were three lines of development arising from Codaster, Troostocri/nts, 

 and Nucleocrijius respectively. Stages in the evolution of these lines determine 

 the establishment of families. 



MOKOCYCLICA. 



DICYCLICA. 



INADUNATA. 



k 



Cambrian. {LARVIFORMIA). 



FLEXIBILTA^ 



lUPINNATA 



(DISTINCT A) 



* - FINNATA^ 



(ARTICULA TA). ' 



OidoTician. 



Silumn. 



Devonian. 



Carbonifeioiu. 



Permian. 



Trias. 



Jurassic. 



Cretaceoua 



Tertiary. 



Recent 



Supposed Relations of the Orders and Sub-Orders of Crinoidea. 



From forms tbat had acquired pentamerism of the theca, and tbat, apparently, 

 possessed hydrospires, the Crinoidea were differentiated by the evolution of true 

 brachia. These in their origin are not exothecal, but actually thecal, and bear 

 along with them epithecal branches of the subvective system, but not brachioles. 

 Such a form is Hyhocystis, Wetherby non Benson. The many-branched and pin- 

 nulate arms, the simple and definite calycal system, or the compound and indefi- 

 nite cup, the plateless or the vaulted tegmen, can all be traced back to such a 

 simple ancestor through actual known genera. The sole gap that cannot be 

 bridged within the limits of the Crinoid class is that between the monocyclic and 

 dicyclic base. The history of these two divisions is shown in the annexed diagram, 

 from which, if it be a true presentment, it appears that the Orders previously 

 accepted are polyphyletic, and are statements of convergent structure, not of 

 affinity. It is, however, well to accept existing terms so far as possible. 



