836 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The young of Comatilia have never been observed until after tlie loss of the 

 larval column and the disapj)earance from external view of the basals. At this 

 stage five large strong orals are present, surrounded by small irregular plates; just 

 above the apposed lateral edges of the radials in the interradial angles of the (•al\-x 

 arc live largo rounded iiiterratlials of equal size which have not to any extent under- 

 gone resorption (figs. 528, 529, pi. 2). These probably have been developed in 

 their j)resent position as m the case of those of Corruictinm. 



Ordinarily these plates never develop further, but soon become resorbed. 

 In two genera, however, ProTnachocrinus and Thaumatocrinus (figs. 113, 114, p. 181, 

 115-118, p. 183, and 505, p. 371), they rapidly increase in size and gradually take on 

 all the characters of the radials between which they are situated, at the same time 

 giving rise to series of plates which form arms in no way distinguishable from the 

 arms arising from the five true radials. 



Anal X lies directly over the posterior basal, always to the left of the radianal 

 and always, if the radianal is present, maintaining a closer relation with the radial 

 to the left of the posterior interradial area than with that to the right, with which 

 the radianal is associated. Whereas the radianal is always a single plate, anal x 

 commonly forms the base of a short series of more or less similar plates. The 

 characteristics of anal x m the fossil forms are naturally assumed by all the inter- 

 radials in the recent types in which interradials are present, for in the recent species 

 the anal interradial is in no way different from the other four. 



The fact that the single linear series of simple plates arising from anal x in 

 many fossil crmoids appears as a complete post-radial series on the homologue of anal 

 X, and on all the other interradials in the recent forms, calls for a word of explanation. 



In the fossil forms the outer border of anal x is far below the dividing line be- 

 tween the ventral surface of the disk and the lateral surface. Tliis lateral perisome 

 is the surface in which anal x itself is formed; therefore, as new areas of perisome are 

 exposed beyond anal x, new plates similar to it will continually be formed, each 

 limited in its breadth by the necessity of providing for motion in the perisome on 

 either side of it, a necessity not operative in the case of anal x which connects two 

 radials and therefore forms the sixth link in the closed radial circlet. 



In the recent Proinuchocrinus and Thaumatocrinus the interradials are from 

 the very first equal in height to the radials, and the next two ])lates are equal in 

 height to the IBr, and IBrj, respectively. 



In the crinoiils the development of a plate after its formation depends not 

 so much upon its previous phylogenetical history as upon the relation which it 

 bears to the three zonal divisions of the skeleton forming dorsal surface, (1) the 

 coronal area, in which the coronal plates, the infrabasals and the basals, occur; (2) 

 the intermediate area in which the radials, division series and first two brachials are 

 formed; and (3) the dorsoventral border line, from which arise the free undivided 

 arms, these being made up in part of an extension from the second zone, and in part 

 of an extension from the ventral perisomic surface. 



Thus the radianal of the fossil species, if developed within the basal ring, becomes 

 a true coronal plate in no way different from the other coronal plates; but in the recent 



