554 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The growth of the topmost columnal as seen in this species apparently is not 

 the preliminary stage of its transformation into a centrodorsal, but instead indi- 

 cates its transformation into a deep cup, within which, concealed from external 

 view, the centrodorsal is developed. At a relatively late stage, when the centro- 

 dorsal has attained considerable size, the basal ring becomes lifted off of the 

 pentalobate cup into which the topmost columnal (as seen in external view) has 

 been transformed, and the centrodorsal, hitherto concealed, becomes visible. 



The structure of the centrodorsal as seen in the largest specimen is much finer 

 than that of the columnals immediately beneath it, indicating a much slower 

 development. It most certainly could not have arisen from a plate resembling 

 the preceding columnals. 



The occurrence of a proximale smaller and of less diameter than the preceding 

 columnals, which it is necessary to assume in explaining the origin of the centro- 

 dorsal in Promachocmnus, is not at all unusual. In recent types such a proximale 

 is found, for example, in Rhizocrinus and in Calamocrinus. 



Terminal stem plate. The terminal stem plate of the pentacrinoid of Proma- 

 chocrinus is at first a circular disk. This soon becomes broadly pentalobate, and 

 the five lobes gradually extend themselves into five long tapering fingerlike 

 processes, so that in its perfected form the terminal stem plate resembles nothing 

 so much as a specimen of Henricia sanguinolenta. 



Infrabasals. It is very difficult to determine the presence or absence of infra- 

 basals with certainty. There appear, however, to be five of equal size, forming 

 a circlet just above the topmost columnal. 



In the earlier stages the lower border of the basal ring shows five rounded 

 or more or less pointed notches situated on the interbasal sutures; these represent 

 infrabasals. Later these disappear, and five notches are formed, each of which 

 occupies the middle of the lower border of a basal. These notches accommodate 

 the processes arising from the distal growth of the interradial portions of the 

 topmost columnal. 



Basals. The basal cup in Promachocrimts is at first noticeable on account of 

 its height and narrowness, being about as long as broad distally. It subsequently 

 expands very rapidly, and in the later stages it becomes unusually broad. 



Radianal. The radianal seems to be the first of the radially situated plates 

 to put in an appearance, and from the evidence at hand we are justified in assum- 

 ing that it is of considerable size at the time the right posterior radial is first 

 formed. 



At first these two plates lie side by side at exactly the same level, the radianal, 

 which is situated on the right-hand end of the very narrow perisomic strip separat- 

 ing the posterior oral from the posterior basal, being accommodated by the cutting 

 away of the distal right-hand corner of the latter. 



The radianal increases in size very slowly and always shows a much finer 

 structure than the other plates. 



The right posterior radial, growing much more rapidly than the radianal, 

 is at first only able to develop on its right side, the development of the left side 

 being prevented by the presence of the radianal. 



