CLARK: PLATES IN CRINOIDS 313 



Anal x in the fossil forms may be reduplicated in the form of a 

 series of interradials, one in each of the other interradial areas; 

 and it therefore does not surprise us to see the same thing in the 

 recent comatulids. 



Sir Wyville Thomson notes that in one or two cases he observed 

 about the time of the first appearance of the anal (radianal mihi) 

 a series of five minute rounded plates developed interradially 

 between the lower edges of the oral plates and the upper edges 

 of the basals. These are strictly homologous with the interradials 

 of the so-called Thaumatocrinus, and that in the posterior inter- 

 radius is anal x. 



In this connection it is most interesting to examine the figure 

 published by Mr. Frank Springer (Journ. Geology, 14: No. 6, 

 1906, pi. 5, fig. 9; explanation p. 493) to show the probable primi- 

 tive structure of the anal interradius and adjacent parts of the 

 calyx in the whole Flexibilia type, both fossil and recent. If 

 we should carry backward to its probable inception the course 

 indicated by the migration of the radianal plate in the young of 

 the recent comatulids we should arrive at a calyx structure iden- 

 tical with that shown by Mr. Springer and deduced from the 

 study of the fossil forms; with the possible trivial difference that 

 we should be inclined to assume the reduplication of anal x in 

 the shape of interradials in all the other interradial areas, though 

 from the data acquired from the study of six-rayed forms it 

 would seem that we would be justified in considering these four 

 additional interradials as a later development. 



Mr. Springer has shown (Journ. Geology, 14: 496. 1906) that 

 in the Flexibilia there is a very pronounced tendency shown by all 

 the radial structures to turn toward the right; the radianal orig- 

 inates under the right posterior radial; from this position it 

 migrates upward until it disappears, always to the right of the 

 median line; if the arms have an asymmetrical distortion it is 

 toward the right, never toward the left; the vertical series of 

 plates arising from the anal x is affected by this tendency, which 

 persists long after the radianal has disappeared. 



In the comatulids the radianal follows the same course as in 

 a succession of fossil genera; the anal tube is always to the right 



