REVISION OF PALEOZOIC STELLEROIDEA. 195 



development of small interbrachial pieces will tend to keep the 

 inframarginals beside the adambulacrals. This is the case in Schuch- 

 ertia where the axillary marginals are also retained, but this genus 

 has progressed considerably beyond this stage, since the inframar- 

 ginals fail to develop in the later growth, while the supramarginals 

 and radials, as columns, are no longer distinguishable as such in 

 any part of the individuals. The tendency of departure from Hudson- 

 aster therefore seems to be toward the breaking up and elimination 

 of its essential structure into a mass of variously arranged small 

 pieces, except the ambulacrals, which remain as columns of stout 



plates. 



Genus SCHUCHERTIA Gregory. 



Plates 32 and 33. 



Palasterina BILLINGS (part, not McCoy or Salter), Geol. Surv. Canada, Can. Org. 



Rem., dec. 3, 1858, p. 76. 



Schuchertia GREGORY, Geol. Mag., dec. 4, vol. 6, August, 1899, p. 351. 

 Trentonaster STURTZ, Verb, naturh. Ver. preuss. Rheinl., etc., vol. 56, 1900, pp. 



224, 225 (based on same genoholotype as Schuchertia). 



Original description. "Palseasterinidse with tessellate exoskeleton; 

 the rays are short and the general shape is pentagonal. R: r as 2:1. 

 Interradial plates numerous, polygonal. Adambulacral plates large 

 and hexagonal. Ambulacral furrow tapering uniformly toward the 

 distal end. 



" Allied to Palseasterina by the general form of the body and the 

 absence of the marginal plates." 



Emended description. Rays five, short and slender. General 

 form pentagonal. Disk large, with well-developed interbrachial 

 arcs. 



Abactinal area composed of very numerous, small, tumid, non- 

 stellate plates which are arranged in quincunx or columns. There 

 are no radial, supramarginal, or ambital plates recognizable as such, 

 nor is the margin bounded by columns of inframarginals. 



Madreporite indistinguishable amongst the abactinal plates. 



Actinally the ambulacral furrows are very narrow, bounded by 

 columns of adambulacral plates. Adjoining these in the proximal 

 regions are distinct " axillary marginals" and inframarginals, the 

 latter distally becoming smaller, and these continue either closely 

 adjoining or are separated from one another by increasing interspaces. 

 In the latter case they are not readily distinguished from the inter- 

 brachial accessory plates. 



Interbrachial areas occupied by numerous small accessory plates 

 and like those of the abactinal area with which they are continuous. 



Ambulacral plates apparently slightly alternating. 



Genoliolotype (of both Schuchertia and Trentonaster). Palasterina 

 stellata Billings. The generic description is, however, largely based 

 on S. laxata, new species. 



