62 SMITHSONIAN INSTITUTION 



rounded arm; the lower part sometimes rests against the left posterior radial, 

 on which a groove is formed by the continual pressure and movements due to 

 the flexible calyx (text-fig. 8). This armlike series is connected with, or rather 

 seems attached to, the pliant integument filled with small, irregular plates 

 which studded the perisome, or ventral covering, in the group. As I have 

 shown in the tegmen of Oiiychocrinus, the perisome developed between the 

 radials and the orals, carrying the latter relatively inward until, instead of 

 covering the whole ventral side as in the larval Antedon (PI. A, fig. 4), they 

 occupied only a small space in the center of the disk, into which the ambulacra 

 converged after traversing the perisome (PI. LXVII, fig. 7). In the present 

 modification of the anal side it would seem as if the perisome began to grow 

 with a similar energy between the rays, so that it encroached upon the anal 

 plate on either side as far as the posterior basal; while the upward extension 

 of that plate took the form of a simple vertical series of rounded plates. Hence 

 this armlike row of anal plates is usually found, in well-preserved specimens, 

 connected with the brachials of adjacent rays upon one or both sides by a 

 bordering integument of irregular perisomic plates (PI. LXVII, figs. 2b, 3b, 

 4, 5). This plated integument, in addition to being pliant, was also very 

 fragile, and its preservation in the fossil state is uncertain and irregular. 



Sometimes the rays lie so close together that they touch the anal series on 

 both sides, and the bordering integument is thus folded inward so that it can- 

 not be seen from the exterior. Nevertheless, the anal plates usually preserve 

 their rounded appearance, and do not seem to form part of the calyx wall, or to 

 be suturally connected with contiguous rays. Such cases as this give rise to 

 difficulties of interpretation, because there are some forms in which the anal 

 plates rise into a single vertical series connected by suture with the adjacent 

 brachials, and flush with them. These must be distinguished from the cases 

 just noticed, where there is a rounded, armlike row of plates, so closely crowded 

 by the rays that there is neither any part of the integument visible, nor any 

 vacant space on either side, but which were evidently not joined to the brachials 

 by suture. Both cases would answer the description " anals in a vertical 

 series," and yet they undoubtedly represent the two distinct plans, which here, 

 as elsewhere, run into perplexing transition forms. 



The armlike row of plates, which are not tubular but on the contrary 

 thick and solid, is the dorsal support of an anal tube formed by the outward 

 growth of the perisome attending the extrusion of the rectum. If in descrip- 

 tions we sometimes call this row of plates " tube-plates," or the " anal tube," it 

 must be understood as a conventional term — used by many former writers in 

 this sense— to avoid circumlocution, and not as implying that the plates them- 

 selves are hollow, or strictly form the wall of a tube. 



