BASAL PLATES IN CRINOIDEA CAMERATA 549 



developed secondarily in the radial cycle, or has migrated into that 

 position. 



The origin of the anal plate is as yet an unsettled question, but 

 there are several possibilities which may well be considered. It 

 may have originated as a secondary plate in either the basal or 

 oral cycle. It may have developed as one of a brachial series, or 

 an interbrachial cycle, or it may have originated as a separate plate 

 in the radial cycle. Origin in the basal or oral cycles is clearly out 

 of the question, for we know of no plates in either cycle from which 

 it could have originated. Assumption of origin as a brachial, which 

 is Bather's explanation for the origin of the anal plate in the Fis- 

 tulata, 1 is without foundation in the Camerata. 



Origin as one of an interbrachial series of the ordinary type is 

 also improbable, for although these interbrachials are present in 

 Tanaocrinus, Xenocrinus, and Compsocrinus, they are clearly formed 

 at a later stage of development. Origin as a first interbrachial, 

 that is, one of an interbrachial series interpolated between the 

 radial plates, has been seriously considered by Carpenter 2 in a 

 comparison of Xenocrinus, and some of the dicyclic Camerata, 

 with Antedon and Thaumatocrinus renovatus; and while such a 

 cycle may have existed and the lateral plates have atrophied, there 

 has been found no record of such a cycle in the monocyclic Came- 

 rata. This, however, does not preclude the idea that the anal 

 series may have been so interpolated, and that the lateral plates 

 which appear in some of the dicyclic Camerata have been the 

 result of reduplication. 3 Let us examine the ornamentation in the 

 anal series of Compsocrinus and Xenocrinus, and see if this may 

 not throw some light upon the question. Bather, in calling atten- 

 tion to the anal ridge in the Reteocrinidae, Glyptocrinus, etc., says, 

 " The anal ridge is connected with the ridges that unite the posterior 

 basal to the right- and left-posterior radials, and this indicates 

 that an axial cord passed up it to govern the motions of the anal 

 tube." 4 That this ridge does so indicate the presence of an anal 

 nerve seems beyond question, for in comparing the ornamenta- 

 tion in Xenocrinus and Compsocrinus with the nervous system of 



1 Ref. 4, pp. 319-31. 3 Ref. 16, p. 338. 



2 Ref. 10, pp. 38-46. 4 Ref. 6, p. 119. 



