23 



Basals three, exposing externally a large heptagonal disc, the 

 side on which the azygous plate rests being short and truncation 

 slight. In the center there is a concave depression for the inser- 

 tion of the head of the column which is surrounded by a pen- 

 tagonal rim, too delicate for illustration, that indicates the column 

 is pentagonal. First primary radials a little wider than long, su- 

 perior face a little concave, three hexagonal and three heptagonal. 

 Second radials more than twice as wide as long, hexagonal. 

 Third radials twice as wide as long, pentagonal, axillary and 

 bearing upon each superior sloping side secondary radials, giving 

 to the species twelve arms. 



There are three regular interradials in each area. The first 

 hexagonal, longer than wide and succeeded by two plates in the 

 second range, each of which is as large as the first one. These 

 unite with the plates of the vault and those covering the ambu- 

 lacral grooves. There is no boundary line between the plates of 

 the calyx and vault as they graduate into each other in the in- 

 terradial areas and the ambulacral coverings. First azygous plate 

 pentagonal, rests on a single basal, and longer than wide; two 

 plates in the second range which reach as high as the calyx 

 proper and unite with the plates of the vault and ambulacral cov- 

 erings. The azygous area is nearly perpendicular from the top of 

 the first azygous plate to the summit of the vault, and, above the 

 second range of plates, is covered by numerous smaller ones. 



Vault ventricose, radial areas high. The larger plate occupies 

 the summit and bears a short central spine or conical elevation. 

 It is surrounded by eight plates, which are next in size, on the 

 vault, all of which are convex and some of them somewhat coni- 

 cal. The other plates on the vault, including those on the radial 

 and interradial areas, are polygonal, convex and granular. The 

 azygous opening is separated from the central plate by three of 

 the eight plates above described, as surrounding the central plate. 

 It is broken, in our specimen, but does not appear to have ex- 

 tended higher than the central plate. 



The specific name is in honor of Prof. A. G. Wetherby, to 

 whose skill and untiring energy is due the discovery of this and 

 the three preceding species as well as the locality from which 

 they were obtained. 



Found in the Trenton Group, in Knox county, Tennessee, and 

 now in the collection of Wm, F. E. Gurley. 



