MORPHOLOGY 5 1 



ambulacra (PI. LIII, fig. ib) ; it occupies the space from the oral pyramid to 

 the margin of the disk, and extends downward and outward between the rays 

 and their divisions where it may connect with well- formed plates which we call 

 interbrachials; usually the distinction between the two is plain, but sometimes 

 they pass insensibly into one another. In either case, however, they are but 

 modifications or extensions of the same element, the perisome; this is primarily 

 a naked skin, but may be partly or wholly studded with calcareous spicules, or 

 with undifferentiated plates, which may become larger or more definite and 

 permanent in the interradial areas; or they may be entirely resorbed, as hap- 

 pens in the tegmen and interrays of most of the living comatulids (See Clark, 

 Monograph of the Existing Crinoids, p. 61, fig. 2; p. 6j, figs. 15-19). Their 

 transient existence in these Recent forms represents a stage of development 

 whose fixation paleontologically produced the heavily plated vault of the 

 Camerata, and the solid interradial areas of the Sagenocrinidae. The relations 

 of this element will be further considered under the next head. 



Fig. 8 



The anal tube in Onychocrinus ulrichi; showing its relation to adjacent parts, and 

 especially the vertical groove in left posterior radial caused by contact and movements of the 

 tube due to the great flexibility of the calyx. 



The anal tube in the Flexibilia is formed by the extrusion from the tegmen 

 of the perisome of the posterior interradius, pushed upward by the rectum into 

 a conical tubular protuberance behind the large posterior oral, and usually 

 supported by a vertical row of strong plates on its posterior side only, as is 

 more fully described later in the generic discussions of Taxocrinus and Onych- 

 ocrinus. The anus is, according to my view, strictly a tegminal opening, 

 analogous to that of the comatulids, no instance of art opening below the level 

 of the radials being known in this group. The genera Sycocrinus and Cydon- 

 ocrinuSj which Bather refers to the Taxocrinidae, have such an opening, but I 

 do not regard them as belonging to the Flexibilia. There is in this group no 

 such elevated ventral sac as is found in the Fistulata division of the Inadunata. 



