384 PROCEEDINGS OF THE ACADEMY OF [1890. 



the radials are in lateral contact. The anal plate of Actinocrimis 

 thus occupies a position toward the radials similar to that of the 

 plate X in Cyathocrlnus, and the anal plate of Melocrinus to that 

 of the plate t (X of Bather) in Hyhocrinus. "We have not the least 

 doubt that the plates of the two former genera are homologous with 

 one another, and also those of the two latter among themselves, but 

 we go no further. We believe that in Melocrinus and Hyhocrinus 

 the plate X is unrepresented, and the structural changes that took 

 place afterwards in Actinocrinus and Cyaihocrintis ^vere not effected 

 by a sinking of the tube, but by the introduction of an additional 

 plate. 



We now return to Dendrocrinus, and take up those forms in 

 which the lower section of the posterior radial turns into a supple- 

 mentary anal piece. The first step in this direction is shown by 

 Homocrinus, Botryocrinus, Oticocrinus and JBarycrimis, in all of which 

 the upper or arm-bearing portion of the radial has shifted slightly to 

 the right, and is connected with the lower by an oblique suture. 

 Here the plate X is comparatively large, occupying the whole space 

 between the two posterior radials, and as no part of the ventral tube, 

 as yet, is introduced into the dorsal cup, the plate R' is physiologi- 

 cally and morphologically a radial, and not an anal plate. 



Bather explains the evolution that took place in these forms by 

 " the shifting of the radianal," and that " X has sunk still lower into 

 the dorsal cup and is now on a line with the radials," in which we 

 cannot agree with him. We have already pointed out that the 

 radianal throughout this group retains the same position which it 

 held when constituting a part of the radial. It always rests with its 

 lower angle between the two basals, and only changes its outlines so 

 as to conform to the adjacent plates. The widening of the anal area 

 was effected by the shifting of the radials, which in Dendrocrinus 

 and Homocrinus opened out for the insertion of the plate X, and 

 by the increase in width which took place in the posterior basal 

 simultaneously. There was no shifting of the radianal, nor any 

 sinking of the plate X, but the latter retained the position which it 

 held in Dendrocrinus. 



In Parisocrinus, Atelestocrinus, Enspirocrinus, and in most of the 

 Poteriocrinidae, the width of the anal area is increased again. In 

 these genera, the arm-bearing section of the right posterior radial 

 moved away from the lower section so as to be placed almost 

 directly upon the right postero-lateral basal, and a new plate was in- 



