NEW RESEARCHES 83 



solidly calcified arms have replaced the tentacles, and short stumps of cirri 

 have appeared upon the proximal columnal. Thus all the various elements of 

 the crinoid skeleton are now represented, with the single exception of the 

 pinnules. Figure 8b is a lateral view of the original of figure 80, showing the 

 extreme distortion of the calyx caused by the bulging of the gut. 



The stage represented by figures la, b, of Plate C is of special interest as 

 showing the condition reached by the oral plates. These now form a low 

 pyramid around the mouth, reduced by relatively one-third from their former 

 size, and disconnected not only from the basals by the large radials which have 

 closed into a complete ring, but from the radials themselves by the extension 

 of a band of interradial plates which tend gradually to pass over into the peri- 

 some. It is substantially the stage represented by the tegmen of Holopus 

 (PL A, fig. 8), of the young Thaumatocrinus (fig. gb), and of the adult 

 Ptilocrinus (PL LXXV, fig. ic). 



From this time on the resorption of the orals rapidly proceeds, and con- 

 currently with it is seen a growing differentiation between the interradials and 

 the ventral integument, which is more and more transformed into undiffer- 

 entiated perisome. The resorptive process also involves the radianal, which 

 has now been left far behind by the protruding anal tube, and is actually as 

 well as relatively diminished in size; the same relative decrease is true of all 

 the dorsal elements of the calyx, which have become nearly flat and collectively 

 serve merely as a shallow saucer for the support of the visceral mass. 



These conditions are shown by figures 20, b and 3a, b, of Plate C, in which 

 the pentacrinoid is almost ready to cast off its stem, prehensile cirri having now 

 developed to a length of 7 or 8 segments, with strong terminal claws suitable 

 for fixation; pinnules of considerable size have also made their appearance; 

 the ventral perisome, still further expanded, is now studded with a multitude 

 of indefinite plates, from which the interradials are well differentiated though 

 reduced in number and evidently in process of resorption; ambulacral grooves 

 have appeared,' traversing the perisome from the arms to the mouth, and pass- 

 ing in between the or,als which are now reduced to very small triangular plates 

 (PL C, fig. 36), the posterior plate in this view being hidden by the anal tube 

 which has become still more conspicuous and rises high above the general level 

 of the disk. The radianal is now entirely above the radial circlet and no longer 

 forms a part of the dorsal cup; the perisome at this stage is covered with a 

 sort of velvety coating in which the numerous small plates are difficult to dis- 

 tinguish, and it is evidently on the verge of transformation into the granular 

 skin of the adult stage from which all plates ventrally situated, including inter- 

 radials and radianal, have been completely resorbed. The stage thus repre- 

 sented by figures 30, b of Plate C is identical with that of the Paleozoic 

 Onychocrinus, but lacking the greatly enlarged and madreporic posterior oral, 



