PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 171 



more elongate. The radianal is seen to encroach upon the oral, the proximal part of 

 which has been partly resorbed and is partly covered by the radianal. The radial to 

 the right of the radianal is asymmetrical, the side adjoining the latter being 

 less developed. 



In the next stage represented, the arms have grown to some length and consist of 

 some 5 or 6 brachials; the exact number of brachials can not be made out distinctly 

 because they are somewhat overlapping and have no such prominent central ridge as the 

 columnals. The radials, which are still widely separated from each other, have assumed 

 a characteristic cordate shape, while the IBr series remain slender. The orals are 

 widely separated from the basals; they have a very characteristic shape, with a deep 

 furrow down the middle line, the sides being gracefully bent outward, as is also the basal 

 portion. The stem is now composed of 27 columnals. The 5 proximal columnals are 

 very short, but wider than the rest, with a prominent central annulus; then follow 2 

 equally short, but much narrower, columnals. The eighth is slightly longer, the next 

 about twice as long, and from the tenth onward they have assumed their definitive 

 shape. 



In the fully developed pentacrinoid the orals have, as a result of the expansion of 

 the disk, shifted their position so that they now lie entirely on its ventral side separated 

 from the calyx proper. The anal cone has developed in the space between the radianal 

 and the adjoining oral so that there is now a large plate both on the outer and on the 

 inner side of the anal cone. The radials have enlarged considerably, and are in contact 

 by their lateral edges. The IBr, has broadened, and the IBr 2 has assumed its character- 

 istic shape with the two oblique articulating surfaces. The brachials are short and 

 broad, somewhat thickened at the ends. The first pinnule to appear is on the twelfth 

 brachial. 



In the younger of the two pentacrinoids at this stage the stem consists of 29 co- 

 lumnals, in the larger of 33. As the cirri are forming on the proximal columnal of the 

 younger, it is evident that no more columnals will be formed. Some of the proximal 

 columnals, from three to five, are broader than the remainder and apparently remain 

 quite short. While in the columnals of the young pentacrinoids the central annulus is 

 wider than the rest of the ossicle and thus appears as a prominent ring about its center, 

 the fully formed columnals are slightly constricted centrally and the central aunulus 

 appeai-s only as a more or less distinct line about the middle. The ends of the columnals 

 are a little broadened. The articulating surfaces are alternating, giving the impression 

 that the columnals are united in pairs, but sometimes the surfaces at either end of a 

 columnal may lie in the same plane. The columnals are smooth and short, the longest 

 only about 0.25 mm. hi length. 



The terminal stem plate is small and slightly lobed. 



Dr. Mortensen says that both the brachials and pinnulars begin as a simple trans- 

 verse rod from which extensions grow out on each side very soon uniting to form a 

 cribriform plate in which the original rod may be distinguished for some time, although 

 it is not nearly so conspicuous as the median annulus of the columnals. 



The first cirri are radial in position, as hi Hatkrometra sarsi and in Tropiometra 

 picta (i.e., carinata carinata). Dr. Mortensen maintains, in opposition to Dr. W. B. 

 Carpenter, that in Antcdon bifida the first cirri are also radial. 



Dr. Mortensen found a detached individual with only two pinnules developed and 

 a third just forming. There are only 8 cirri, the 5 of the primary whorl and 3 of the 



