210 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



gigantic Democrinus weberi, very near in stem structure, though vastly inferior in 

 size of cro\TO, to Phrynocrinus nudus, as well as in individual columns in the 

 species of Naumachocrinus (fig. 129, p. 201). 



The transition from the primitive antedonid or bourgueticrinoid tj-pe of stem 

 to the curiously twisted column of Platycrinus may he easily followed in a good 

 series of the young of certain of the species of that genus, or even in single speci- 

 mens in which the young stem is preserved. Certain species of Platycrinus when 

 fully growii appear to lose the chstal portion of the colunm in just the same way as 

 the same thing occurs in the pentacrinites, though in Platycrinus the free existence 

 seems to be assumed somewhat later in hfe, and in many species is never assumed 

 at all. 



I have observed the change from the Antedon-Uke young stem to the radiaU}' 

 arranged adult stem in Isocrinus and in related genera (fig. 143, p. 205), and have 

 noticed that in the largest species of BatJiycrinus the fulcral ridges of the articula- 

 tions broaden out on each side of the central canal, becoming more or less wedge- 

 shaped or triangular, and exliibiting a strong tendency to break up into radiating 

 ridges, the articulations thus approacliing the uniformly rachated type found in 

 such genera as Calamocnnus, Proisocrinus (fig. 525, pi. 1), PtUocnnus, Ilyocrinus, 

 Oephyrocrinus, and Thalassocrinus so closely as to leave no doubt as to the 

 possibility of their origin in tliis way. 



It might be urged that the articular faces of the colunmals of the pentacrinites 

 and of the upper part of the stem in Proisocrinus and Carpenterocrinus, with their 

 petaloid markings, could not be placed in the same class with articulations hke tliose 

 of Calamocrinus, where the joint faces are uniformly marked with radiating lines; 

 but in these genera it is merely a case of the colunmals, primaiily with articular 

 faces bearing regular rachating lines, being molded or cast into petaloid sectors 

 by the under surface of the basals against wliich they he and against wliich they 

 are formed, these basals being in a curiously reduced condition, between the normal 

 type of basal as seen in Calamocrinus or in Ptilocrimis, and the atropliied and 

 metamorphosed condition seen in Antedon, though more closcl}- approacliing the 

 latter. In Proisocrinus, indeed, all types of cohmmals occur from those with 

 radiating ridges upon the joint faces, at the base of the stem, to those with petaloid 

 sectors, just under the cal3-x (fig. 128, p. 199). 



In the pentacrinites and in certain species of Platycrinus the earliest part of 

 the column, as abeady explained, is just hke the stem of the young comatulid; this 

 never develops further, but is eventually discarded, much as the stem is discarded 

 in the comatulids. In Proisocrinus, however, the young stem is not discarded, but 

 develops along the hues mdicated m the large species of Bathycrinns and Rliisocrinus 

 until the Calamocrinus type is reached. Probabty when young Proisocrinus ])os- 

 sesses basals like those of Ptilocrinus or of Calamocrinus; in later hfe, however, the 

 basals gradually become dwarfed, or at least do not develop in ]>r()j)ortion to the 

 other calj'x elements, so that thoj' approach m character those of the ])entacrinite3, 

 and with this change m the basals the columnids also Ijegin to assume the pen- 

 tacrinito form. 



