PALEONTOLOGIC CONTRIBUTIONS 47 



ambulacrals that leave space for large podial pores and meet in a 

 well-defined, straight, ambulacral channel or gutter. The adara- 

 bulacrals are typically coin-shaped, the narrow ridges are some- 

 what obliquely directed forward and inward and separated by fur- 

 rows about as wide as the ridges. The ossicles number about 

 twenty in 10 mm, and there are about thirty in a column. The 

 first three adambulacrals diminish in size to the basal plate, which 

 with the neighboring one bears a very minute triangular syngnath 

 or mouth angle plate, The adambulacrals also bore two or three 

 spines or rods each. 



Horizon and locality. Arisaig series (Silurian) at mouth of 

 Stonehouse brook, Nova Scotia. 



Remarks. We have two specimens of this Urasterella before us, 

 one showing the actinal and the other the abactinal side. Both were 

 collected by Doctor Clarke. The actinal side agrees in its outline, 

 size, number and character of adambulacral ossicles in a certain 

 space with the figures published of U. ruthveni (Forbes) 

 from the Upper Ludlow rocks near Kendal, Westmoreland, Eng- 

 land. This species, the genotype of Urasterella, is not well known 

 and its abactinal side has not yet been described. Positive identifi- 

 cation is therefore at present impossible. As, however, the 

 Arisaig series has so many forms in common with the European 

 Silurian, it is highly probable that this Arisaig type is conspecific 

 with the English form. The Arisaig series has been shown by 

 Schuchert and Twenhofel to represent a period of time in Europe 

 between the lower Llandovery and the Ludlow. As the English 

 species comes from the Upper Ludlow, it is very probable that our 

 species represents an earlier mutation and a direct identification 

 would be liable to mislead. 



Urasterella lutheri sp. nov. 



Plate 13, figures 2-6; plate 14, figures 1-4 



Description. Disk relatively small, about one-fourth the size of 

 the animal, rays of very flexible appearance, fairly long and robust. 

 Abactinal disk area covered by a great number of very small, 

 irregularly stellate ossicles without regular arrangement that are 

 separated by deep intervals; many (or all?) of these bore a central 

 rod or mucro ; on the rays the abactinal side is composed for the 

 most part of four-rayed stellate ossicles that are distinctly arranged 

 in columns, seven to nine of which can be counted on the mature 

 part. The rays of the ossicles touch only at their tips, thus leaving 



