REVISION OF PALEOZOIC STELLEEOIDEA. 219 



Genotype and only species. Palxocoma cylindrica Billings. 

 11 Remarks: This genus differs from PalseopTiiura by the absence 

 of marginal ossicles from the disk, and from Sturtzura by the smaller 

 size of the disk and the form of the syngnaths. In Sturtzura the 

 jaws end bluntly against a jaw plate, whereas in Tseniura they appear 

 to taper to a point and have no jaw plate." 



The two cotypes of T. cylindricus (1405a) were lost for many years, 

 but have recently been rediscovered in the collections at Ottawa. 

 In addition, there is another specimen in that Museum under this name, 

 and it is probable that it was so identified by Billings (No. 1405). 

 This was the only specimen studied, and its generic characters agree 

 very well with the originals of the cotypes (figs. 4b and 4c of pi. X), 

 and appear to be in harmony with the genotype of Tseniaster. The 

 only important difference is that here the ambulacralia are directly 

 opposite one another, and not slightly alternating, as in T. spinosus 

 (see fig. 18), a difference, it seems to the writer, not in itself alone 

 worthy of being the base of a new genus distinct from Tseniaster. 

 Tseniaster has the following species: 



T. spinosu^s (Billings). Lower Trenton. 



T. cylindricus (Billings). Trenton. 



T. schofiarize Ruedemann. Upp^r Trenton. 



T. elegans Miller. Richmondian (Waynesville). 



T. meafoadensis Foerste. Richmondian. 



TXENIASTER SPINOSUS (Billings). 



Plate 36, fig. 1; text fig. 17. 



Palseocoma spinosa BILLINGS, Geol. Surv. Canada, Rep. Progress for 1853-1856, 



1857, p. 292. 

 Tseniaster spinosus BILLINGS, Geol. Surv. Canada, Can. Org. Hem., dec. 3, 1858, 



p. 81, pi. 10, figs. 3a-3d. MILLER, N. Amer. Geol. Pal., 1889, p. 285, fig. 439. 



Revised description ly BiUings. "The specimens collected are 

 about seven lines in diameter; the rays linear-lanceolate, one line 

 in width at the base, and covered at the sides with numerous small 

 spines. 



"In the view of the enlarged specimen (pi. X, 3s), the ambulacral 

 ossicles appear in some places to alternate with each, other, but this 

 is owing to a distortion. Those on one side of the furrow are oppo- 

 site those upon the other. The adambulacral plates are elongated, 

 and so placed that the outer extremity of the one lies upon the inner 

 extremity to the next. The rays are flexible." 



For remarks on this species, see the discussion under Tseniaster. 



Formation and locality. Two specimens from the Lower Trenton, 

 Falls of Montmorency, east of Quebec, Canada. The original of 

 figures 3a to 3d is in the Victoria Memorial Museum, Ottawa 

 (No. 1404). 



