56 SMITHSONIAN INSTITUTION 



b. The Anal Structures 



Modification of the plates of the posterior, or anal, interradius, taken in 

 connection with the posterior basal, form the basis of the two largest divisions 

 of the Flexibilia ; the details of these two plans of structure are fully explained 

 later. A series of further and somewhat parallel modifications occurred under 

 each of the sub-orders, which furnish the characters for several of the genera. 

 The first of these modifications relates to the plate now called by authors gener- 

 ally the " Radianal.'' The morphological relations of this plate, and its signifi- 

 cance in classification, were not understood in reference to the Flexibilia until 

 I pointed them out in my paper of 1902, 1 in which I demonstrated the existence, 

 not before suspected, of such a plate in the well-known species " Taxocrinus " 

 tuberculatum from the Silurian of Dudley, England, and proposed for it on that 

 ground the genus Temnocrinus; and in that of 1906 2 in which I proved the 

 same fact, equally unsuspected, in the Silurian species of Ichthyocrinus, and 

 proposed to separate from them the Carboniferous species, in which no such 

 plate is present, under the name Metichthyocrinus. Subsequent investigation 

 has abundantly confirmed the conclusions there announced, and has extended 

 them to four other genera, so that the structure of the anal side in the Flexi- 

 bilia in this respect is now established by a multitude of facts and made per- 

 fectly clear. 



The term " Radianal " (RA) was first proposed by Bather 3 in 1890 for a 

 plate which is prominent in the development of the anal interradius of the 

 Inadunata. As the name implies, it stands partly in connection with the radial 

 system, in this case with the right posterior radial, and at the same time it 

 functions as a plate eventually leading toward the anal series. How intimate 

 the latter connection is I am now able to demonstrate far more clearly than has 

 ever been done before, as will be seen a little further on. 4 



This plate may be described as occupying four positions, to which may be 

 added by way of a Hibernian concept a fifth, its absence: (1) At the base of 

 the right posterior ray and in contact with the right anterior radial — its primi- 

 tive and oldest position (Cupulocrinus, Dendrocrinus, etc.) ; (2) under the left 



1 American Geologist, vol. 30, p. 88. 



~ Journal of Geology, vol. 14, p. 475. 



3 Ann. and Mag. Nat. Hist., ser. 6, vol. 5, p. 329. 



* My discussion of the position and modifications of the radianal is limited to the Flexibilia, in connec- 

 tion with the development of a corresponding plate in the larval stages of some Recent crinoids, and with 

 illustrations drawn from the dicyclic Inadunata, from which it is now thought the Flexibilia may have 

 been derived. Whether the lower segment (inferradial) of the compound right posterior radial in the 

 Heterocrinidae and allied monocyclic forms should be regarded as homologous with the radianal, as has 

 at times been done by authors (myself included), I shall not undertake here to discuss. The fact that 

 some of these genera have similar compound radials in other rays, and that the true anal series only begins 

 on one face of the axillary superradial, constitute formidable objections to that view. The homologies of 

 this and the anal plate x, in the Crinoidea as a whole, will be more fully discussed in a paper entitled 

 "What is the Radianal?" which is now being prepared for publication by my assistant, Dr. Herrick 

 E. Wilson. 



