496 CLARK: CRINOID FAMILY PLICATOCRINIDAE 



tulids before the appearance of the final stem segment, which 

 eventually will come to form the centrodorsal. 



12. There are no cirri; the column is attached by a heavy ter- 

 minal stem plate as in the young of the comatulids. 



A few words of explanation regarding the column of the Articu- 

 lata may not be out of place here. The column of the Articulata 

 (entirely absent in the Comatulida Innatantes, the one suborder 

 of comatulids not represented in the recent seas) is entirely differ- 

 ent from that of any other crinoids for, instead of growing continu- 

 ously thruout life thru the formation of columnals just under the 

 crown, it possesses a definite growth limit at which further increase 

 in the number of columnals abruptly ceases, and the last columnal 

 to be formed becomes attached to the calyx by close suture (usu- 

 ally fusing with the inf rabasals) , enlarges, and becomes the so- 

 called proximale which is, to all intents and purposes, an apical 

 cal.yx plate. Immediately below this enlarged columnal or prox- 

 imale is another more or less modified columnal to which it is at- 

 tached by a modified close suture, the so-called stem syzygy 

 which, except for a superficial resemblance, has nothing in common 

 with the brachial syzygies. 



The typical form of the column in the Articulata is seen in the 

 young of the comatulids at the time of the formation of the cen- 

 trodorsal; but this typical form persists in the adults only in the 

 genus Thioliericrinus, and in the family Phrynocrinidae. In 

 . such types as the Apiocrinidae the proximale is so enormously 

 enlarged that it involves with itself in this process a considerable 

 number of the columnals beneath it, so that a cone shaped series 

 of enlarged columnals is formed, each of which is a reduplication, 

 progressively Jess perfect, of the proximale just beneath the calyx. 

 Essentially the same state of affairs is seen in the Bourgueticri- 

 nidae, especially in the recent genus Ilycrinus. In the comatulids 

 (excepting in the Innatantes, which never possess one) the column 

 is discarded at the stem syzygy between the proximale and the 

 columnal just beneath it. 



In the young pentacrinite the proximale is formed exactly as in 

 the young comatuUd; but the great excess of column growth in- 

 duces the formation of new columnals between the proximale and 



