DISCOVERY OF THE DISK OF ONYCHOCRINUS 495 



As I have shown by the tegmen of Onychocrlnus, 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 very early Antedon (Plate V, Fig. 3), they occupied only a 

 small space in the center of the disk, into which the ambulacra con- 

 verged after traversing the perisome (Plate IV, Fig. i). In the present 

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

 to grow with similar energy toward the dorsal side and down 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, bordered upon one or both sides by small, irregular plates, 

 which were a part of the perisomic integument lying between them 

 and the adjacent rays (Plate IV, Fig. 5). This plated integument, in 

 addition to being pliant, was also very fragile, and its preservation 

 in the fossil state is very uncertain and irregular. Sometimes it 

 evidently fell to pieces when exposed between the rays, and no trace 

 of it is found preserved; sometimes it was folded deeply inward 

 between the rays, and so covered with matrix that it has not been 

 observed in the fossils. This has often led to misconception of the 

 real structure, and to misrepresentation of the facts in illustrations. 

 For instance, it was at one time a matter of dispute whether the genus 

 Taxocrinus, as then supposed to be represented by T. tubercidatiis 

 from Dudley, possessed any interbrachial connection between the 

 rays, beyond a possible single plate. Proper cleaning now discloses, 

 in many specimens of that species, the integument of small plates 

 rising high up between the rays and their divisions. The charac- 

 teristic appearance of this armlike series of anal plates, and also of 

 the perisomic integument, is well shown in Wachsmuth and Springer's 

 figures of Taxocrinus intermedius,'^ and also in the various ligures 

 of Onychocrinus herein (Plate IV). 



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 cannot be seen from the exterior. Nevertheless, 



I Proceedings, Academy of Natural Science, Philadelphia, November, 1888, 

 Plate XVIII. 



