58 SMITHSONIAN INSTITUTION 



primitive position of a right posterior infer radial by a migration to the left and 

 upward to final complete elimination as a calyx plate in the Carboniferous. 



The fact that in some Silurian genera represented by Sagenocrinus and 

 Homalocrinus, the radianal entered the basal ring through the widening to a 

 variable extent of the right posterior interbasal suture, does not in my opinion 

 alter the conclusion that the order of succession in the migration of this plate 

 was from below upward. The earliest position, viz., that in Protaxocrinus, 

 Cupulocrinus, etc., near the beginning of the Ordovician, was at the base of the 

 right posterior ray, and this continued throughout the Silurian. The inclusion 

 of the radianal within the basal circlet, as in Sagenocrinus and Homalocrinus, 

 may be interpreted as occurring during a process of shifting, not from above 

 downward but from the right toward the median area, in course of which it 

 came into the posterior interray, the line of weakness of the Flexibilia calyx. 



This shifting process was in fact an incident of the migration of the plate 

 from the first to the fourth position, during which temporarily and irregularly 

 the two basals — posterior and right posterior — yielding to the influence of the 

 gut pushing from right to left, opened to a varying extent, and admitted a cor- 

 responding growth of the radianal in their direction. The great irregularity 

 of this movement is well shown in Sagenocrinus, in some specimens of which 

 the radianal rests nearly to its full width upon the infrabasals, in others meets 

 them by only a short line of contact (PI. XVIII, figs. 2.0, 6), or failing to 

 touch them rests upon the sloping faces 1 of the two basals (PI. XIX, fig. 5) 

 precisely as in Lecanocrinus. This last case is in fact a transition stage afford- 

 ing definite proof that the tendency of the radianal within the limits of the 

 genus itself was really upward. Furthermore, in this genus there is a marked 

 tendency of the interbrachials in all the areas to grow downward as well as 

 upward, so that in a number of instances the first interbrachial is in contact 

 with the basal. Such an irregular downward growth of the radianal cannot 

 therefore be said to represent any well-defined phase of its migration toward 

 the lower oblique position, which became definitely established in several well- 

 marked Silurian genera of the Flexibilia, and thence to the upper oblique posi- 

 tion which persisted as a leading character in some of their strongest genera 

 to the close of the Carboniferous. 



The position of the radianal in its Carboniferous stage at the upper right 

 of the unsymmetrical posterior basal is shown by numerous examples illus- 

 trated upon the plates, both in forms with and without the anal tube. In 

 Forbesiocrinus it appears under almost every species — for example Plates 

 XXIII, figures la, 2; XXV, figure 2; XXVII, figure 8 — where it occupies a 

 place in the line of least resistance directly above the lower oblique position in 

 such forms as Lecanocrinus and Pycnosaccus, the space occupied by the large 



