Agelacrinidae and Lepadocystinae 
489 
interambulacral plates. There is a tendency toward a ridge par- 
allel to the longer diameter of the plate in case of the bordering 
plates, along the sides of the rays. There is no evidence of pores 
or of a madreporite. 
Thresherodiscus differs from all other Edrioasteroidea, hitherto 
described, in the presence of branched ambulacral rays, and in the 
very pronounced trimerous origin of these rays. The strong- 
differentiation between a central series of large squamose imbri- 
cating interambulacral plates and the smaller bordering interam- 
bulacral plates is noteworthy. 
The presence of a single row of large floor plates is an interesting 
feature, but is known also in Agelacrinus cincinnatiensis, A. hol- 
hrooki, A. piieus, A.faheri, and A. austini. 
It is evident that Thresherodiscus finds its nearest relatives 
among the Agelacrinidae, but it probably had quite a different 
origin from the Ordovician species usually referred to Agelacrinus. 
31. Agelacrinus vetustus, sp. nov. 
(Plate III, Fig. l) 
For ten years I have had in my possession a specimen of Agela- 
crinus which is of interest chiefly because it was found in the Green- 
dale or richly fossiliferous member of the Cynthiana formation, 
on the south side of the Kentucky Elver, at Clays Ferry, 14 miles 
southeast of Lexington, Kentucky, opposite the southeastern 
corner of Fayette County. It occurred in the fossiliferous strata 
between 38 and 69 feet above a massive limestone layer, near a 
road side watering trough. The specimen is attached to a pedicel 
valve of Rafinesquina, and apparently is covered by the thin, 
densely papillate stroma of some Dermatostroma, which obscures 
the outlines of all of the thecal plates excepting those belonging to 
the ambulacral series, and even here tubercles are found on the 
outer, exposed faces of the lateral covering plates. This papillate 
stroma does not extend from the theca of the Agelacrinus on to the 
surface of the Rafinesquina, upon which it rests. In fact, for some 
reason this stroma does not actually reach the extreme margin of 
the theca but is separated from the latter by a very narrow space 
along which the vertical ridges belonging to the outermost rows of 
very small marginal plates are exposed. Ridges of a similar sort 
are illustrated by Hall. Twenty-fourth Report of the New York 
