HEXACEINID^. 785 



completely within the radial facets ; wider than long, sharply angular at the 

 top, their sloping upper faces concave. Arms four to the ray, their structure 

 unknown. Ventral disk highly elevated, resting upon the inflected upper 

 ends of the radials and anal plate, and hence narrower than the dorsal cup ; 

 the sides almost vertical, and the summit flat. The jDosterior oral, which is 

 subcentral, is large and strongly convex, the four others considerably nar- 

 rower and almost flat ; the outer ends of the latter curve abruptly down- 

 ward, and take part in the sides of the disk. The radial dome plates rest 

 at right angles between two orals; they are quite large, and are extended 

 into short, heavy obliquely directed spines. The first range of interradials 

 consists of three pieces, of which the two outer ones are very narrow and do 

 not touch the radials; they are followed by two small interambulacrals, and 

 these by the orals. The anal plate supports upon its truncate upper face 

 a rather large quadrangular piece, and upon its upper sloping faces an elon- 

 gate narrow one, followed by other i'ows of three plates of smaller size, and 

 by a moderate number of other pieces, which form a small protuberance 

 containing the anus. Anal opening on a level with the posterior radial 

 dome plates. 



Horizon and Locality. — Upper part of St. Louis group ; Eussellville, Ky. ; 

 Logan Co., Ky., and Flagpont, Va. 



The ty2:)e specimen, formerly in Dr. Shumard's collection, could not be 

 found in the Museum of Washington University at St. Louis, and is probably 

 lost. The specimens figured are from the collection of Wachsmuth and 

 Springer. 



Remarks. — This species is readily distinguished from T. cornigenis by the 

 greater depression and tumidity of the dorsal cup, the constriction at the 

 arm bases, the erect form of the disk, the narrowness of the anal interradius, 

 and the comparatively small number of disk plates. 



y 



