

MONOGRAPH OF THE EXISTING CEIXOIDS. 339 



5-rayed types, resorbed soon after their appearance instead of developing after the 

 manner normal for the genus. 



In this connection it is most interesting to examine the figure published by 

 Mr. Frank Springer to show the probable primitive structure of the anal inter- 

 radius 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 indi- 

 cated by the migration of the radianal plate in the young of the recent comatulids, 

 we should arrive at a calyx structure identical with that shown by Mr. Springer 

 and deduced from the study of the fossil forms. From the study of the recent types 

 alone it might be argued that the figure should be slightly modified by the redupli- 

 cation of anal x in the shape of interradials in all the other interradial areas; but 

 from the data acquired from the study of 6-rayed specimens, and the very evident 

 modification of all the recent types in the direction of a perfect, derived from an 

 imperfect, radial symmetry, it would seem that we would be justified in considering 

 these four additional interadials as a later development. 



Sir Wyville Thomson believed that the minute interradials sometimes present 

 in the young of Antedon lifida occasionally persisted and became the clusters of 

 small plates often observed in the angles of the calyx in the adult; but it is far more 

 likely, as P. H. Carpenter has suggested, that these latter are secondary perisomic 

 plates, and that the true interradials whenever they appear are either resorbed 

 like the orals or develop into interradial radials. 



Pcrisomic interradials. 



In many of the recent comatulids more or less well-defined plates are found 

 between the division series and between the first two or three brachials of the free 

 arms. These may be comparatively small and distinct, or they may bo large, 

 forming a solid calcareous plating over the perisome. They are most strongly 

 developed in certain of the large very many armed comasterids, as Comastcr multi- 

 fida, C. Mil, C. typica and Comantkina schlegelii, and, though hero restricted to 

 small areas between the bases of the IBr,, are very prominent features of certain of 

 the species of Antedon, especially of A.moroccana undA.diibcnii (fig. 104, p. 167). 



These plates havo nothing to do with true interradials of the typo seen in the 

 young of Promachocrinus, Thaumatocrinus, Comactinia, Comatilia or Antedon, but 

 arise from a calcareous deposition within the more superficial layers of the peri- 

 some. These perisomic intorradials will be considered in connection with the 

 other perisomic plates and the perisomic spiculcs, and in the section dealing with 

 the Pentacrinoid Larvse. 



Primary plates of the disk. 



In the young of Thaumatocrinus renovatus (figs. 115-118, p. 183) the surface of 

 the disk between the margin and the outer border of the orals is completely invested 

 by a pavement of small plates which later disappear, just as docs the radianal. 

 The same development of a complete but transient plating of the disk occurs in the 

 young of Comactinia, the plates here being rcsi.i -lied first on the ventral surface of 

 the disk, and later in the lateral interradial areas. 



