MONOGRAPH OF THE EXISTING CBINOIDS. 555 



At about the stage when the radianal and the right posterior radial are equal 

 in size (two similar plates situated side by side, one, of coarser structure, situated 

 on the line of division between the basals and the orals, the other, of finer struc- 

 ture, lying just to the left of it) the radianal begins to migrate to the left. At 

 the same time, as a result of the growth of the radials, the distal lateral angles 

 of the basals become more and more broadly cut away until the basals, instead 

 of being bounded distally by a straight edge, are bounded by two distal edges 

 which meet in the middle at an obtuse angle. Thus the course taken by the 

 radianal in its migration toward the left along the distal border of the posterior 

 basal, at first horizontal, soon becomes diagonally upward toward the apex of 

 the basal. Meanwhile the right posterior radial is rapidly increasing in size. As 

 a result of the upward, as well as outward, course taken by the radianal, the 

 center of the latter remains on the same level with that of the right posterior 

 radial (the excision of the latter being bounded by a more or less deeply concave 

 line, with the chord more or less vertical) until the left side of the radianal touches 

 or extends slightly beyond the dorsoventral plane, passing through the apex of the 

 posterior basal that is to say, until about the time of the formation of the first 

 brachials. 



From this time onward the course of the radianal is almost directly upward 

 and outward, the proximal left corner of the right posterior radial rapidly 

 extending itself along the distal right border of the posterior basal until it reaches 

 the median distal apex, where it meets the right side of the left posterior radial 

 and completely excludes the radianal from the calyx. 



After its exclusion from the calyx the radianal lies for some time almost 

 or quite directly over the posterior interradial suture before it disappears. 



Radials. Beyond the relatively late union of the two posterior radials beneath 

 the radianal, the radials of Promachocrinus in their development do not differ from 

 those of previously known types. 



Interradials. Interradials have been described in the young of Antedon bifida 

 and of Comactinia jneridionalis. but in these species they are always formed be- 

 vond the distal margin of the radial circlet, and ai'e eventually resorbed. 



Similarly in Promachocrinus the interradials do not appear until after the 

 circlet of radials is complete; but in this type they are formed not beyond the 

 radials but between them, apparently first on the apices of the basals, thence 

 rapidly extending themselves, as very narrow plates, anteriorly until they entirely 

 separate the radials. Broadening rapidly, while at the same time the radials 

 cease their lateral extension and become relatively narrower, they gradually 

 assume all the characters of the true radials, and on the middle of their distal 

 border a small plate appears, soon followed by another, and the latter by two 

 more side by side, until we recognize the beginnings of the interradial arms. 



These interradial arms, which first appear before the formation of the centro- 



dorsal and the disintegration of the orals, increase in size very rapidly, but they 



do not attain the size of the radial arms until the animal is almost fully grown. 



An extraordinary fact in regard to this species is that the interradials do 



not always develop, or only the one in the posterior interradius develops, so that a 



