REVISION OF PALEOZOIC STELLEROIDEA. 177 



Stenaster pulchellus Billings and this is the general consensus of opinion. 

 The writer unfortunately had no material of the genotype of Uras- 

 terella for study. However, since U. grandis is closely related to U. 

 pulchella and shows so much more detail, it is here accepted as the 

 genotype for the time being. 



Professor Hall (1870) wrote: " In Stenaster = Urasterella, however, 

 we have the absence of adambulacral plates." This is nevertheless 

 not the case, for the corn-shaped marginal series in S. pulchellus are 

 the ambulacral plates, since weU-developed inframarginal plates, as 

 in Palseaster, are absent in mature Urasterella. The inframarginals 

 are, however, seen to be present in U. pulchella or in the distal ends 

 of the rays in other species, though proximally they are so modified 

 as not to be readily distinguishable from the ambital plates. In the 

 axils of the young of U. ulrichi the inframarginals are well developed, 

 along with single interbrachial marginal plates, but in mature speci- 

 mens of other species none have been seen. 



Of Roemeraster the writer has seen three excellent specimens skil- 

 fully prepared by Sttirtz which clearly show that this genus has the 

 generic structure of Urasterella. Stiirtz 1 describes this genus as 

 having "strong marginal plates" inside of the "small, cornered adam- 

 bulacrals." Our specimens do not show such plates, and if present 

 they can not be adambulacrals. This fact can be demonstrated 

 in a well-preserved distal end of a ray, where two columns are present 

 on each side of the ambulacrals. The innermost column of closely 

 adjoining plates increase in size, retain iheir position beside the ambu- 

 lacrals, and become the most prominent of the entire animal, while 

 the outermost column soon change in aspect, the plates become 

 separated and assume the characters of the ambital pieces. This 

 is likewise true in the young of U. ulrichi where the axillary areas 

 also have distinct interbrachial marginal and two larger proximal 

 inframarginal plates. Passing distally, the latter also soon change 

 then- form, are separated one from another and are not distinguish- 

 able from the ambital plates. Stiirtz further describes and figures 

 five interbrachial ossicles in mature Urasterella asperula which are 

 also well developed in the young of U. ulrichi. Urasterella is an 

 easily recognized genus and the triangular ambital plates drawn out 

 into short blunt erect rods are peculiar to it, a character again re- 

 peated in Roemeraster. Under these circumstances the latter appears 

 to be indistinguishable from Urasterella. For other remarks see 

 U. asperula. On the other hand, Gregory regards Roemeraster as a 

 good genus and places it erroneously among the ophiuridlike asterids 

 in the family Tseniasteridse. 



i Verb, naturh. Ver. preuss. Rheinl., etc., vol. 50, 1893, p. 52. 



