2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 93 



of an interray with a hydropore located between it and the mouth. 

 These primitive echinoderms, free or attached by a part of the lower 

 surface, developed into parasitic circular flattened forms, into elevated 

 sacklike bodies or into cylindrical objects bearing the ambulacra at 

 the top of a fused mass of plates. Starting at the left of the anal 

 area and proceeding clockwise, the ambulacra are designated as follows : 

 I, left posterior ; 2, left ; 3, anterior (opposite the anal area) ; 4, right ; 

 5, right posterior. These numbers are employed in the following 

 descriptions. 



Through the study of the Springer and Ulrich collections of Edrio- 

 asteroidea in the National Museum, and of material lent by other insti- 

 tutions, the writer has been enabled to review the structure of most of 

 the known genera and species of this group, the generic results of 

 which are ofifered below. Here, the old genera are briefly redefined and 

 six new generic names are proposed. This paper is introductory to 

 a more complete publication that will include descriptions of additional 

 new species upon Avhich this classification is based. 



Class EDRIOASTEROIDEA Billings, 1854-58 



(Thyroidea Chapman, i860; Agelacrinoidea S. A. Miller, 1877-1883; Cys- 

 tasteroidea .Steinman, 1888; Thecoidea Jaekel, 1895; Cystostellaroidea Steinman, 

 1904.) 



Family AGELACRINITIDAE, new name 



(Agelacrinidae Jaekel, 1899; Thecocystidae Jaekel, 1899) 



Theca flexible, composed of thin plates attached temporarily or 

 permanently by the greater part of the aboral surface and with am- 

 bulacra confined to the oral surface. A single row of ambulacral 

 flooring plates overlapping proximally. 



STROMATOCYSTITES Pompeckj, 1896 

 (Stromatocystis Bather, 1900) 



Theca depressed, pentagonally globular in form with oral surface 

 bearing five narrow, straight ambulacra separated by polygonal mosaic 

 interambulacral plates with numerous intervening pores and the under 

 surface completely occupied by smaller, less regular polygonal plates. 

 Anal area slightly elevated, indistinct. 



Genotype. — S. pentaiigularis Pompeckj, 1896. Middle Cambrian of 

 Bohemia. S. balticits Jaekel, 1899, S. zvalcoffi and var. minor Schuch- 

 ert, 1919 are other species of the genus. 



