202 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



rigid column, and not accomplished after the calcareous deposition has been com- 

 pleted and the rigid character attained. 



No recent crinoids are known in which the first line of development obtains; 

 but it is well illustrated by the fossil genus Edriocrinus. No crinoids are known 

 in which the column is composed simply of two columnals, as would be the case in 

 the first stage in the second line of development. But suppose we carry this line 

 further; we have a crinoid attached by a column in which an articulation has been 

 developed in the center; such an articulation would of necessity develop a fulcral 

 ridge running across the joint faces and embracing the central canal, admitting of 

 motion in a single plane, coinciding with that hi which the original stimulus deter- 

 mining the fracture was received. Stem growth would continue; but as new 

 deposition occurs only just under the calyx, only the proximal columnal would 

 increase in length. Soon the proximal columnal would become so long as to become 

 brittle, as did the original stem, and fracture would again occur midway between 

 the first articulation and the calyx. Now, this fracture would almost certainly 

 differ from the original fracture in being formed at right angles to it, for any force 

 exerted in the same plane as that which caused the original fracture would be taken 

 up by the articulation which has formed; but, owing to the definite direction of, 

 and the close union along, the fulcral ridge, any force coming parallel to the fulcral 

 ridge that is, at right angles to the original force would meet with resistance, as 

 for a force exerted in this direction the original articulation would be practically 

 nonexistent, and a second fracture would occur in the weakest spot; namely, half 

 way between the original articulation and the calyx, developing into a second 

 articulation in which the fulcral ridge would run at right angles to the direction 

 taken by that of the first. A still further increase in stem length would mean a 

 progressive increase hi the number of articulations, each of which would, hi the 

 direction taken by its fulcral ridge, alternate with those on either side; anil thus 

 would eventually be formed the primitive polycolumnar crinoid stem, a stem 

 exactly comparable to the stem of Rhizocrinus (figs. 135, 137, p. 205), Baihycrinus 

 (fig. 527, pi. 2), and the young of the comatulids (figs. 407, p. 317, 532, 533, pi. 3). 



Although the origin of the polycolumnar crinoid stem appears undoubtedly to 

 to have been from a single original calyx plate, a centrale corresponding to the 

 centrale in Marsupites (fig. 565, pi. 7) or in Uintacrinus (fig. 572, pi. 7) and to the 

 central plate of certain echinoids, it does not necessarily follow that the redupli- 

 cation of the col um rials was the result of a series of actual morphological fractures 

 as just described. 



This is the most obvious explanation, and the one which may be most readily 

 grasped; at the same time, through explaining the development of the alternating 

 fulcral ridges, it indicates with a reasonable degree of accuracy the method by which 

 the rapidly developing columns of the later fossil and of the recent types, as opposed 

 to the slowly developing columns of the palaeozoic forms, have come into existence. 



The primitive type of column, occurring in the palaeozoic species almost 

 exclusively, but persisting in the recent Plicatocrinidfe, is characterized by short 

 cylindrical coluninals which have the articular faces marked with radiating 

 ridges. The explanation of the origin of this type of column is somewhat 



