Agelacrinidae and Lepadocystinae 
431 
of the substomial chamber. On the left side of this quadrangular 
plate, near the junction between the lateral margins of the proxi- 
mal floor plates belonging to the right and right posterior rays 
(Nos. 4 and 5), there is a broad inclined groove. 
It has been found impossible to unravel the structure of the 
peristomial parts of the theca from the view of its under surface, 
as exposed within the substomial chamber as viewed from below. 
Apparently this peristomial part consists of a number of small 
plates instead of a few larger ones, but no definite structure has 
been recognized. Moreover, no apertures leading from the ambu- 
lacral grooves along the median parts of the rays, beneath the 
modified floor plates, as seen from below, into the substomial 
chamber, have been detected. Under these circumstances, it is 
not worth while to speculate regarding the use of the deep depres- 
sion or aperture at the posterior margin of the substomial chamber, 
or the purpose of the inclined surface or broad groove on the 
antero-lateral side of the quadrangular plate filling the left posterior 
side of the substomial chamber, as seen from below. Possibly 
the gut twisted in a solar direction, and left the substomial chamber 
along this inclined surface, but the direction of the gut is unknown 
in the Agelacrinidae. 
Description of Several Ordovician Agelacrinidae 
29. The Use of the Generic Terms Agelacrinites 
AND LePIDODISCUS 
Clarke has shown {New Agelacrinites, 1901, p. 193) that the 
type of Agelacrinites, the Devonian species Agelacrinites hamil- 
tonensis,is characterized by the presence of mosaic interambulacral 
plates, with irregular polygonal outlines, and a more or less radial 
sculpture; also by narrow rays, and a well developed peripheral 
ring consisting of an inner band of large plates and a marginal zone 
of small plates. 
From typical Agelacrinites, the Ordovician species formerly 
referred to Agelacrinus, merely a different spelling to the same 
generic term, differ in the presence of imbricating interambulacral 
plates, and wider rays. It should be noted also that the floor 
plates of Agelacrinites beecheri are narrower, and evidently support 
only one lateral covering plate on each side, instead of being 
