INTERRELATIONSHIPS 6 



O 



Now the position of this row of plates shows in a striking manner the 

 effect of that singular influence which, as pointed out by me in 1906, 1 has modi- 

 fied the bilateral symmetry of almost every genus of this entire group. The 

 small infrabasal is almost invariably located under the right posterior ray; the 

 radianal originates under the right posterior ray; it migrates from this posi- 

 tion upward until it disappears, but always to the right of the median line; if 

 the arms have an asymmetrical distortion, it is to the right, never to the left. 

 If the posterior basal is angular above, the right sloping face is the longest 

 (PI. XXIII, figs, la, 2). This vertical series of anal plates is influenced by 

 the same tendency, and the position of its lower plate probably represents one of 

 the later stages of the radianal. The posterior basal on which it rests is ex- 

 cavated into a shallow socket, like the articulating facet of a radial, on the 

 right shoulder of the plate, so that we usually see a small tongue or angle of 

 that plate rising up to the left of the base of the anal plate higher than to the 

 right (Pis. LII, fig. 10; LIII, figs, la, 2b). This asymmetry in the position of 

 the socket-like excavation is variable in degree, but even where by reason of 

 pressure of the adjacent rays the tube appears to be median, careful examina- 

 tion will usually show that the exposed part of the basal is slightly widest at 

 the left. Not only so, but the anal series itself, even when the lower plates 

 touch the left posterior radial, eventually leans to the right, so that the vacant 

 space, or the plated integument if it is preserved, is always widest at the left; 

 and the anal plates are sometimes found firmly cemented by pressure to the 

 side of the right posterior ray, leaving no other plates visible at that side, thus 

 giving an appearance of sutural union which must be guarded against in 

 studying these fossils. 



The shape and position of this anal row of plates are subject to much 

 variation owing to pressure in fossilization, but the dextrorse asymmetry is 

 almost the invariable rule. This is due to the line of weakness at the right of 

 the posterior interradius which is followed by the radianal in its upward 

 migration. 



After citing the foregoing statement from my paper of 1906, in the dis- 

 cussion already referred to, 2 Mr. Clark says : 



In the ontogeny of the comatulids the radianal follows the same course as in a succession 

 of fossil genera ; the anal tube is always to the right of the median line of the posterior inter- 

 radius ; that the supplementary arm arising on anal x in the young of Thaumatocrinus 

 renovatus and of Promachocrinus kerguelensis does not turn to the right is to be interpreted 

 purely as a secondary condition, the result of its origin on the edge of the disk and its free 

 extension outward from the body. Were the series of ossicles following anal x in the young 

 of Thaumatocrinus and Promachocrinus incorporated in the perisome, we cannot doubt but 

 that it would have followed the anal tube in its migration to the right, and would, therefore, 

 have come into complete correspondence with the conditions seen in the fossil Flexibilia. 



'Journal of Geology, Vol. XIV, p. 496. 



' Monograph of the Existing Crinoids, 1915, p. 332. 



