314 CLARK! PLATES IN CRINOIDS 



of the median line of the posterior interradius; that the supple- 

 mentary arm arising on the anal x of Thaumatocrinus renovatus 

 does not turn to the left is to be interpreted purely as a secondary 

 condition, the result of its origin on the edge of the disk and its 

 free extension outward from the body. Were this series of ossi- 

 cles following anal x in Thaumatocrinus incorporated in the peri- 

 some we cannot doubt but that it would have followed the anal 

 tube in its migration to the right, and would therefore have come 

 into complete correspondence with the conditions seen in the fossil 

 Flexibilia. 



In Thaumatocrinus renovatus the disk between the margin and 

 the orals is completely enclosed by a pavement of small plates 

 which later disappear, and the same is true in certain other forms. 

 This heavy plating of the disk in the very young of species of 

 which the adults have naked disks must be of very profound signi- 

 ficance, and, when taken in connection with the occurrence of 

 the radianal and anal x, and with various other features, strongly 

 indicates that it is the transient vestige of the forerunner of the 

 dome of the Camerata. 



SUMMARY 



1. The so-called anal in the pentacrinoid larvae of the recent 

 comatulids is in reality the radianal of the fossil forms. 



2. Anal x is represented in the pentacrinoid larvae of the com- 

 atulids by a posterior interradial which gives rise to an additional 

 post-radial series, as in Thaumatocrinus renovatus and in six-rayed 

 specimens of other species, or by a minute plate which is quickly 

 resorbed; in the recent forms it is repeated in all the interradial 

 areas. 



3. Thaumatocrinus renovatus is merely the young of Decame- 

 trocrinus abyssorum (P. H. Carpenter). 



4. The solid plating of the disk which appears in the young of 

 certain forms concurrently with the disappearance of the orals, 

 quickly to be resorbed, is the transient vestige of the condition 

 which developed into the solid dome of the Camerata. 



