EEVTSIOiSr OF PALEOZOIC STELLEROIDEA. 169 



concerned, are closely related. The difTcrenccs, however, are worthy 

 of generic rank. In Stenaster the rays are petaloid, with the adambu- 

 lacrals largest at the mid-length of the ra3's, while in Tetraster the 

 rays are triangular, with the greatest ossicles in the axils. 



At first the genotype looks as if one had a direct representative of 

 living Phancrozonia, but a little study quickly shows that the large 

 actinal margming plates are not marginals or inframargmals, but 

 adambulacrals, and therefore indicate a cryptozonian form. For a 

 while it seemed that these large plates must be inframargmals, but 

 then adambulacrals should be present, and they were looked for 

 along the vertical ambulacral walls, but even hero there are no 

 diminutive pieces of these columns. Among Paleozoic asterids the 

 writer has not seen a form without adambulacrals, and when there are 

 but four actinal columns it is always the inframarginals that are 

 diminutive or absent. 



This species may be identical with T. (?) imhricatus (Salter), 

 which see for further remarks. 



Distribution. — Restricted to the Upper Ordovicic at Thraive, Gir- 

 van, Scotland. The specimens occur as very sharp natural molds in 

 a micaceous sandstone, from which good wax squeezes can be made, 

 revealing all the skeletal characters. The originals are in the collec- 

 tion of Mrs. Robert Gray, Edinl)urgh, Scotland. 



TETRASTER (?) IMBRICATUS (Salter). 



Palxaster imbricaius Salter, Mem. Geol. Surv. Gt. Britain, vol. 3, 1866, p. 

 289, pi. 23, fig. 8. 



Original description. — "P. rigidus, uncialis. Radii breves conici, 

 nee ad basin valde expansi, subcarinati, tubercuHs seu jugis obliquis 

 remotiusculis asperi. 



''Ray conical, its length about five lines, its breadth at base less 

 than two lines. Only two rays are preserved; these are convex, 

 slowly tapering, somewhat carinate a])ove (we do not see the lower 

 side), and ornamented by tubercles gathered into curved oblique 

 rows, which run forward from the avenues toward the carina above, 

 and are separated from each other by a space little more than their 

 own breadth. 



"LocaUty: Caradoc rocks, Llanfyllin, Montgomeryshire [Wales]. 

 The specimen is in the collection of Mr. Prosser." 



Remarlcs. — As the writer has not seen the holotype, his conclusions 

 are based on the lithographic figure, which adds but little knowledge 

 to that in the description. As there are but two columns of dorsal 

 ray plates, it hardly seems that the species is a phanerozonian. In 

 general it looks most like Tetraster, but as the dorsal side of this 

 form is unknown, no direct comparisons can be made between them. 

 On the other hand, the imbrication of the plates, their large size, 



