2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



In the list at the end of this paper only the articles dealing with the 

 Edrioasteroidea in a more extensive way than the description of a 

 species or two are cited. The bibliography of the American Ordovi- 

 cian and Silurian species can be found in the author's Bibliographic 

 Index of 1915. For facility of reference the original generic name of 

 the described species is inserted in parentheses. All the known species 

 of the entire group are mentioned in this or the 1935 paper. 



4 



Class EDRIOASTEROIDEA Billings, 1854-58 



Family STROMATOCYSTITIDAE, new family ] 



Stromatocystites Pompeckj with its basal layer of plates and its 

 allies JValcottidiscus Bassler and Xenocystites, new genus, are so 

 different from the edrioasteroids with basal areas of attachment with- 

 out plates that this new family seems necessary. 



WALCOTTIDISCUS Bassler, 1935 



Theca depressed, hemispherical to pentagonal globular, with five 

 straight or curved ambulacra separated by polygonal interambulacral 

 plates on the oral surface, and aboral surface completely covered by 

 more or less polygonal plates. 



Associated with the genotype, ]V . typicalis Bassler, is a second 

 species possessing the same generic features, notably a theca as in 

 Stromatocystites, but with curved ambulacra, four (i to 4) directed 

 to the left, and one, the right posterior (5), to the right. Better pre- 

 served specimens of this genus are necessary before the exact nature 

 of the base can be determined, although the plate structure here 

 appears to be as in Stromatocystites. 



WALCOTTIDISCUS MAGISTER, n. sp. 



Plate 2, fig. 2 



The type specimen, a subpentagonal, depressed, semiglobular theca, 

 6 cm in diameter, apparently free, is, although crushed in hard shale 

 and imperfectly preserved, sufficient to show that this is a magnificent 

 species characterized by long, narrow, strongly curved ambulacra, 

 four curved to the left and the right posterior to the right. The inter- 

 ambulacral areas are wide and occupied by large, polygonal, slightly 

 imbricating plates, but the anal pyramid cannot be distinguished unless 

 represented by a cluster of several small plates. The edge of 

 the theca seems to be formed by small imbricating plates and 



