BASAL PLATES IN CRINOIDEA CAMERATA $51 



The logical sequence of events based upon the Thaumatocrinus 

 theory would be the interpolation of the anal plate in the radial 

 cycle and serial development of the succeeding anals. But when 

 this sequence of events is applied to the pentamerous, monocyclic 

 Camerata difficulty is immediately encountered. Either these 

 forms have lost their true anal plate and the anal now appearing 

 in them is the homologue of the second anal in the hexagonal 

 Camerata, or they have developed along a different line of evolution 

 from the hexagonal forms. But before taking up these questions let 

 us examine this theory of interpolation more closely. 



Stratigraphically the pentamerous base precedes the hexamerous 

 base, and it seems scarcely possible that a hexagonal form which 

 could give rise to both Glyptocrinus and Tanaocrinus could have 

 been living in the ocean basin during pre-Ordovician and Ordovician 

 times, and that only those forms having lost the anal plate should 

 have migrated into the epicontinental seas during the Ordovician, 

 while those having the anal plate were withheld until Silurian 

 (Richmond) times. This is apparently carrying the theory of 

 selective action too far to be believed. Again, the embryological 

 evidence shown in Thaumatocrinus may not be reliable. We have 

 noted that the radianal in Promacocrinus originates to the left of 

 the right-posterior radial; furthermore, we know that the radianal 

 appears in the more primitive crinoids in a subradial position. 

 There has been not only a progressive upward shifting of the radi- 

 anal in these groups, but there has been apparently a progressively 

 upward shifting of its point of origin. Embryology does not repeat 

 all the ancestral characteristics step by step and then eliminate 

 them in a different fashion in producing the various genera and 

 species; certain characters which are gradually being eliminated 

 phylogenetically are probably, in Crinoidea, the result of a progres- 

 sively increasing inhibition of plate development in the larva, 

 which ends in the complete obliteration of the plate. 1 Since embry- 

 ology does not repeat all the ancestral stages, and does in this 

 case permit of changes in the position of origin of a plate, there is a 

 possibility that the " interradial " radials in Thaumatocrinus did 



1 For a more complete discussion of this phase of development based upon a ^Yide 

 series of observations, see Ref. 23, Chap. Ill, "Recapitulation." 



