164 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Madreporite unknown. 



Adambulacral plates with granular surfaces, few in number, never 

 more than 17 hi a column, subquadr angular and thick, greatest width 

 about the middle of the rays, rapidly diminishing in size distally and 

 making the margin of the rays and disk. The great proximal adam- 

 bulacrals are the orals, occupying the axils in pairs, acutely triangular, 

 more or less pointed, and terminating orally. 



Ambulacral plates large, directly opposite one another and the 

 adambulacrals as well, h- -shaped, with the widest portion in the per- 

 radial center of the rays; there are as many as 16 or 17 ambulacrals 

 in a column. Laterally each plate is more or less excavated, both 

 distally and proximally, thus leaving very large, subcircular podial 

 openings. 



Spines of any kind are so far unknown. 



Genoholotype (the first species and the one selected by subsequent 

 workers). Stenaster salteri Billings. 



Remarks. This genus has not had a good standing, due largely 

 to the fact that Palseaster Hall was poorly known when Billings 

 described Stenaster and further because he included two distinct 

 generic types in his genus. A careful reading of Stenaster, however, 

 reveals that Billings actually based his genus on S. salteri, the first 

 form following the generic description, and this selection has been 

 followed since. Later on, McCoy applied the name Urasterella in 

 a loose manner to the second species (Palseaster pulcJiellus) , and 

 although it is a form of this genus it is not the genotype. Hall (1 868) 

 directed attention to this work of McCoy, but erroneously assumed 

 that all of Stenaster was synonymous with Urasterella. It is true, 

 as stated by Hall, that Billings's second species is congeneric with 

 Urasterella, but the first species and genotype is certainly quite dis- 

 tinct from McCoy's genus. Sttirtz (1890) also points out the same 

 thing and gives a short and emended diagnosis of " Urasterella 

 (Stenaster)." In 1893 the same writer applied McCoy's name 

 Urasterella to Stenaster salteri and made S. pulchellus the genotype 

 of Urasterella, a proceeding that violates the rules of nomenclature. 



Stenaster as here restricted differs from Urasterella in being more 

 decidedly rigid and not so flexible as the latter, while the rays are 

 far shorter and petaloid in form. For these reasons, Stenaster has 

 comparatively few adambulacrals and ambulacrals, while there are 

 many more of these ossicles in Urasterella. Finally, the former 

 reveals no abactinal plates or spines of any kind, while the latter 

 has a skeleton of small plates that terminate in long inarticulate 

 spines or rods. 



Actinally Stenaster is closely related to Tetraster, and the only 

 marked difference is that the former has petaloid rays and more 

 pronounced adambulacral oral jaws. 



