REVISIOX OF PALEOZOIC STELLEROIDEA. 165 



There would be no need to point out h^re the difference between 

 such distinct genera as Stenaster and Palseaster were it not that 

 Hall regarded the former as a synonym of the latter. In Stenaster 

 the rays are petaloid and not slender and regularly tapering, as in 

 Palxaster. The most marked distinction, however, is the fact that 

 these two genera belong to different orders, for Palseaster has columns 

 of inframarginals that are completely absent in Stenaster. Further, 

 in the last-named genus, the interbrachial areas are mad(^ up each 

 of two pau'S of oral armature pieces, while in Palxaster there are, 

 distally to these, additional large smgle axillary interbrachials. 



Stenaster and the related genus Tetraster are very primitive in 

 construction. Ac finally they are seen to have the structure of 

 Hudsonaster minus the inframargmal columns and the single axiUary 

 interbrachials. It is this difference that makes the former genera 

 members of the order Cryptozonia, while Hudsonaster is the most 

 primitive of Phanerozonia. Until the abactmal area is Imown in 

 Tetraster and Stenaster, positive phyletic relationship can not be 

 stated, but in a number of specmiens of the latter genus the evidence 

 at hand indicates decided specialization through the complete removal 

 of the heavily plated skeleton of Hudsonaster. 



Distribution. — Ordovicic of xAjnerica. The following are the 

 known species: 



S. salteri Billings. Black River and Trenton. 



S. (?) ohtusus (Forbes). Bala of Ireland and Wales. 



S. (f) coronella (Salter) . May Hill sandstone (Siluric) of England. 



S. (?) conjiuens Trautschold. Upper Carboniferous of Russia. 



STENASTER SALTERI Billings. 



Plate 32, fig 1. 



Stenaster salteri Billings, Geol. Surv. Canada, Can. Org. Rem., dec. 3, 1858, 

 p. 78, pi. 10, figs, la, 16. — Wright, Mon. British Foss. Ecliinod., Oolitic, 

 vol. 2, pt. 1 (Palseontogr. Soc. for 1861), 1862, p. 28, fig. 15 (2) on p. 24. 



Original descrijition. — "Tliis species has rather short, broad rays, 

 which are narrower where they are attached to the very contracted 

 body than they are at about the center of their length. In conse- 

 ciuence of this form, the sides of the rays are not parallel, but a little 

 curved outward. As however only two specimens have been col- 

 lected, and both appear to be a little flattened by vertical pressure, 

 it may be that this leaflike shape of the rays is accidental [an 

 abundance of specimens show this fomi to be the natm'al one]. 

 * * * The adambulacral plates are oblong [granular and from 7 

 to 17 in each colmnn in the smallest and largest examples], and 

 the sutures between them are nearly at right angles to the ambu- 

 lacral grooves; those next the body are a little sloping outward. 



