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 assiuned 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 entu-ely 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 thei'e 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 IBrj has broadened, and the IBrj has assumed its character- 

 istic shape mth 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 yomiger of the two pentacrinoids at this stage the stem consists of 29 co- 

 lumnals, in the larger of 33. As the cirri are fonning on the proximal columual 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. WhOe 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 colmnnals are slightly constricted centrally and the central annulus 

 appears only as a more or less distinct fine about the middle. The ends of the columnals 

 are a little broadened. The articulating sm'faces are altei'nating, 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. in 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 wliich the original rod may be distinguished for some time, although 

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



The fu'st cirri are radial in position, as m Hathrometra sarsi and in Tropiometra 

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

 Carpenter, that in Antedon bifida tlie first cirri are also radial. 



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

 a third just formuig. There are only 8 cu-ri, the 5 of the prunar}' whorl and 3 of the 



