210 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



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

 size of crown, to PTirynocrinus nudus, as well as in individual columns in the 

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



The transition from the primitive antedonid or bourgueticrinoid type of stem 

 to the curiously twisted column of Platycrinus may be 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 grown appear to lose the distal portion of the column 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 life, and in many species is never assumed 

 at all. 



I have observed the change from the Antedon-like young stem to the radially 

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

 noticed that in the largest species of Baihycrinus 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 exhibiting a strong tendency to break up into radiating 

 ridges, the articulations thus approaching the uniformly radiated type found in 

 such genera as Calamocrinus, Proisocrinus (fig. 525, pi. 1), Ptilocrinus, Hyocrinus, 

 Gephyrocrinus, and TTialassocrinus so closely as to leave no doubt as to the 

 possibility of their origin in this way. 



It might be urged that the articular faces of the columnals 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 like those 

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

 but in these genera it is merely a case of the .columnals, primarily with articular 

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

 by the under surface of the basals against which they lie and against which they 

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

 type of basal as seen in Calamocrinus or in Ptilocrinus, and the atrophied and 

 metamorphosed condition seen in Antedon, though more closely approaching the 

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

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

 sectors, just under the calyx (fig. 128, p. 199). 



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

 the column, as already explained, is just like 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 lines indicated in the large species of Bathycrinus and Rlvizocrinus 

 until the Calamocrinus type is reached. Probably when young Proisocrinus pos- 

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

 basals gradually become dwarfed, or at least do not develop in proportion to the 

 other calyx elements, so that they approach in character those of the pentacrinites, 

 and with this change in the basals the columnals also begin to assume the pen- 

 tacrinite form. 



