106 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



for its origin may be had other than that it represents an entirely 

 new element. One of these crinoids is of special interest as showing 

 a complete fusion between the centrale and certain of the infrabasals. 

 Tliis fusion quite parallels that to be found in the case of certain 

 Uintacrinus. 



One may only speculate as to the conditions under which an eleu- 

 therozoic habit was assumed by Arachnocrinus. It would appear 

 that detachment from the column took place naturally but perhaps 

 not universally. However such may be, we know that the form lived 

 in the comparatively shallow waters about coral reefs where either 

 an excessive shortening of the column or its complete elimination 

 would tend to be induced. Subsequent to its acquisition of freedom 

 it is probable that Arachnocrinus crawled about on the bottom rather 

 than swam. The comparatively small size of the theca and the rela- 

 tively enormous expanse of the massive arms seem to preclude the 

 possibility of active swimming movements on the part of this organism. 

 A full discussion of these remarkable crinoids will be given in a forth- 

 coming work. 



Having discussed the features more intimately associated with the 

 different genera referred to this type, and having shown the presence 

 of a centrale in certain normally attached forms, it may be well to 

 recapitulate the evidence relative to the formation of this plate. 

 This seems best as in the discussion of various tj^pes such facts have 

 been scattered and used to illustrate specific structures. 



Upon the loss of the column by a form in which a persistent proxi- 

 mal columnal is not present or in which specialization has not brought 

 about a fusion of the proximal columnals into a centro-dorsal, we 

 may have the formation within the proximal circlet of plates of an 

 entirely new element. Subsequently this element may fuse with the 

 adjacent plates of the cup, but this process is but a secondary one 

 and of no great importance. In other cases, as will be pointed out 

 later, upon the loss of the column the axial canal is closed simply by 

 the increase in the size of the elements of the proximal circlet. This 

 shows that the formation of a new plate is not essential. Further- 

 more it points to the possibility that in the case of Saccocoma we need 

 not have had a centrale originally to which the basals fused. Instead 

 the centrale in that form may represent simply the fused basals. 

 At all events the centrale may not be held as of any considerable 

 morphologic significance, and is of no great value in determining 

 relationships. 



It is unnecessary to enter upon a discussion of the homologies of 

 the centrale, inasmuch as being an absolutely new element, it is not 

 homologous with any structures in other crinoids. It may be seen 

 that a consideration of it as representing the "dorso-central" or the 



