REVISIOISr OF PALEOZOIC STELLEEOIDEA. 129 



ossicles. Rays with three columns of plates, one radial and two 

 supramarginal, that may or may not be separated from one another 

 by small accessory pieces. Madreporite dorsal and interradial. 



Genus XENASTER Siinonovitsch emend. Schondorf. 



Plate 24. 



Archxasterias 'MJJLLER,Yerh.. naturh. Ver. preuss. Rheinl., etc., vol. 12, 1855, pp. 

 6, 8 (Archxa^crias founded in error). 



Xenaster Simonovitsch, Sitzb. mat.-naturw. Classe Akad. Wiss., Wien. vol. 64, 

 Abt. 1, 1871, J). 88. — Schondorf, Palseontographica, vol. 56, 1909, pp. 42, 

 105 (complete synonymy given here); Jahxb.nassauisch. Ver. Naturk., Wies- 

 baden, vol. 62, 1909, p. 26. 



Generic characters. — Xenaster has its nearest relationship in 

 Devonaster. No detailed description need be given, since the splendid 

 illustrations of Schondorf give all the necessary characters. It 

 differs from Devonaster as follows: 



In Devonaster there is but a single interbrachial plate in each 

 actinal axil, the axillary interbrachial, while in Xenaster there are 

 two pairs of interbrachial marginals and a single axillary inter- 

 brachial plate; in other words, five interbrachial plates instead of 

 one (1+2+2). In addition to these plates in Xenaster, the interbra- 

 chials are more or less surrounded by small accessory pieces, and then 

 other similar plates separate the inframarginal and adambulacral col- 

 umns to near the outer third of the rays. The inframarginal plates in 

 Xenaster do not increase so rapidly in width proximaily as do these 

 plates in Devonaster, and while the interbrachial areas are of about 

 the same size in both genera, the space occupied by the additional 

 interbrachial marginal and accessory interbrachial ossicles in the 

 former is taken up in the latter genus by the increased size of the 

 mframarginal pieces. 



Abactinally Xenaster has the generic characters of Devonaster. 

 The plates in all the columns of the former genus are more prominent 

 and rounder, particularly the accessory plates, of which there appear 

 to be less than in Xenaster. The center of the disk in both genera 

 is occupied by a small central disk plate, but there are many more 

 accessory plates around it in Devonaster than in Xenaster. In both 

 genera the supramarginal columns converge proximaily, producing 

 small disk areas between the infra- and supramarginal plates occu- 

 pied by ambital accessoiy plates. 



Madreporite large and striate in Xenaster. 



Genotype. — X. margaritatus Simonovitsch. 



