MONOGRAPH OF THE EXISTING CRINOIDS. 335 



for them, in young Antedons of about 2 inches in diameter, he concluded that 

 the entire plate is removed at once by a continuance of resorption over its whole 

 surface. 



Interradials; Anal. 



In the recent crinoids the secondarily perfected radial symmetry has become 

 so thoroughly established that the anal plate (corresponding to the anal x of the 

 fossil forms) is never in any way differentiated from the interradials occupying the 

 other interradial areas. All five of the interradials are either present and developed 

 to exactly the same degree, or all five are absent. 



The so-called anal in the young of the comatulids, a large and important 

 element in the calyx of all the forms in which the young are known, is not in any 

 way homologous with the anal of fossil species, but represents the radianal, which 

 itself is the last remnant of five theoretical primitive subradial plates persisting 

 beneath the right posterior radial; these five primitive subradial plates are them- 

 selves the equivalent of the five entire ambulacra! series of the urchins, except for 

 those plates immediately surrounding the peristomal area, which are represented by 

 the radials. 



The anal x is the equivalent of the second interambulacral plate of the echinoid, 

 the plate immediately following the genital; anal x together with the 'series which 

 commonly follow it are the equivalent of the entire interambulacral series in the 

 urchins with the exception of the genital, which is represented by the basal upon 

 which anal x rests. 



Since the radianal is represented in the pentacrinoids of the comatulids we 

 should expect also to find in the posterior interradius a second plate which we 

 could with a reasonable degree of probability identify as the representative of the 

 plate known as anal x; and such a plate actually occurs. 



Sir Wyville Thomson in one or two cases observed hi the developing young of 

 Antedon lifida at about the time of the appearance of the radianal a series of five 

 minute rounded plates developed interradially between the lower edges of the 

 orals and the upper edges of the ba.sals. These plates therefore separate the radials 

 from each other all around the calyx. They are the equivalent of the five intor- 

 radials in the fossil species, and that in the posterior interradius is the homologue 

 of anal x. 



In the young of Comactinia five interradials of equal size are found; they are 

 late in making their appearance, being first noticeable at about the time when the 

 IBr, are formed. They never grow to a large size, but remain as five rhombic 

 plates in the interradial angles, each about half as long as the basal beneath it. 

 Neither do they rest upon the basals as they do in the young of Pramacliocrinus 

 and Thaumaiocrinus , for the radials have come into lateral contact before their 

 appearance; they thus lie in the angle made by the cutting away of the distal 

 angles of the radials in such a way that a line connecting the bases of two adjacent 

 IBr, would pass approximately through their center. 



In a single instance I found a pcntacrinoid of this species in which there were 

 three interradials instead of the usual one in each interradial area, one between 

 the distal ends of the radials and two side by side just above it (fig. 412, p. 317). 



