MORPHOLOGY 



45 



the plate, while its inner surface corresponding to the face of the socket slopes 

 toward the outer margin of the brachial, thus making the vertical section of 

 the process triangular and its transverse section lunate. The marginal contact 

 surfaces of the process and socket are crenulated, as contact surfaces usually 

 are. Between their apposed faces there is sufficient space for the dorsal liga- 

 ment, which perhaps took on an oblique or a nearly horizontal position. 



When the arms were folded, the dorsal ligament would be stretched and 

 the process would separate slightly from the socket ; but when opened, the liga- 

 ment would contract and pull the process firmly into the socket. Thus there 

 was an interlocking of the brachials which was probably connected with a 

 stiffening of the crown. The relatively feeble articulations in the Flexibilia, 



dl 



7a 



6d 



7b il 



Figs. 6a-d, ya-d 



THE " PATELLOID " PROCESSES 



6a, b, dorsal view of a pair of brachials, showing the projection from proximal edge of 

 upper, and corresponding socket on lower ; 6c, d, ventral view of same, showing perfectly 

 straight sutures, ya, b, c, cross sections of three brachials in different stages, showing shal- 

 lowness of socket and thinness of patelloid process, their progressive development with cor- 

 responding enlargement and change in position of the dorsal ligament fossa, and the deepening 

 of the inter-articular ligament fossa ; yd, distal face of IBr from which the section yc was 

 made — a very mature brachial in which the development of the socket and process has pro- 

 ceeded to the extreme, and the structure of the articulating surface has been modified by age ; 

 pt. patelloid process; dl. dorsal ligament fossa; il. inter-articular ligament fossa. 



combined with the large size of the arms, would render the latter peculiarly 

 susceptible to fracture in case, of contact with a fish or other moving object. 

 With the arms closely coiled, the crown would be compact and in no danger of 

 breakage through an external force; and when extended they would be auto- 

 matically stiffened by the overlapping of the brachials. 



d. The Arms 

 The arms of the Flexibilia are all uniserial, no suggestion of a biserial 

 arrangement being found in the entire group; and they are, as already stated, 

 always multibrachiate, simple arms being unknown. No trace of syzygies has 

 been observed, nor of cuneiform plates, all having parallel proximal and distal 

 faces. In the mode of branching two types prevail: (i) By approximately 



