THE CLASSIFICATION OF THE EDRIOASTEROIDEA 



By R. S. BASSLER 

 Head Curator, Department of Geology, U. S. National Museum 



(With One Plate) 



The Edrioasteroidea, a group of Paleozoic echinoderms, regarded 

 by specialists either as a distinct class of the Pelmatozoa allied to the 

 cystids or as an aberrant order of the same division, has been the 

 subject of so many detailed observations and changes in nomenclature 

 that much confusion exists in its classification. Many of the genera 

 have inexact limits assigned them, because they are based upon incor- 

 rect illustrations or described from species other than the cited geno- 

 type, or, again, because they are founded upon characters of uncertain 

 value. Lastly, some authors, without regard to the rules of nomen- 

 clature, changed the generic endings to suit their ideas as to the 

 relationships of the group, Agclacrinites, for example, becoming first 

 Agelacrinus and later Agelacystis. Furthermore, it must be remem- 

 bered that the camera-lucida drawings of years ago resulted in reversed 

 images, the right side becoming the left, which in the edrioasteroids 

 gave rise to a serious error since the direction of the ambulacra, a 

 generic character now believed to be important, thus was reversed. 

 However, most students paid little attention to the extent and direction 

 of curvature of the ambulacra, including in the same genus forms 

 most diverse in these respects. 



In preparing the present classification the writer first of all proved, 

 at least to himself, from a study of several hundred specimens of 

 several Cincinnatian species, that the amount of imbrication of the 

 interambulacral plates, the width of the peripheral border of plates, 

 and the number of arms or ambulacra may vary even in the same 

 species ; but the plate structure of the ambulacral and oral areas, and 

 the direction and extent of curvature of the ambulacra, remained 

 constant, thus affording good generic characters. 



In their simplest form the edrioasteroids have a flexible theca or 

 sacklike body composed of numerous more or less polygonal plates 

 having a mouth on the upper surface at the central point of radiation 

 of five straight or curved arms or ambulacra separated by interam- 

 bulacral plates, and an anus with valvular covering occupying part 



Smithsonian Miscellaneous Collections, Vol. 93, No. 8 



