Agelacrinidae and Lepadocystinae 
421 
plate is flattened in a direction parallel to the base of the theca, 
and this flattened surface supports the lateral covering plates, as 
already mentioned. 
Such traces of the interambulacral plates as remain are seen 
only from below and are not clearly defined. They, no doubt, were 
small and polygonal, forming an irregular mosaic as in other 
species of Streptaster. 
21. Floor Plates of Thresherodiscus 
In Thresherodiscus ramosa, the floor plates also occur in a single 
row (plate I, Fig. 8; also plate III, Fig. 3). The median third 
of each plate is occupied by a comparatively shallow ambulacral 
groove. The lateral thirds of the plates are comparatively flat. 
These lateral parts probably were grooved longitudinally for the 
articulation of the lateral covering plates, but at present they are 
too weathered to verify the former presence of the lateral grooves 
with certainty. The floor plates overlap each other in a proxi- 
mal direction, when viewed from above. 
Toward the tips of the branches of the ambulacral rays, the 
floor plates so closely resemble the ordinary interambulacral 
thecal plates that it seems certain that the floor plates of Thresh- 
erodiscus are to be regarded merely as specialized thecal plates. 
Some of the rays appear to intrude even upon the upturned 
margins of the adjacent plates belonging to the inner band of the 
peripheral ring. In other words, the ambulacral rays are regarded 
as epithecal. 
From the preceding observations it seems probable that the floor 
plates of all typical Agelacrinidae will be found to be arranged in 
single rows, and to overlap each other proximally, when viewed 
from above. This proximal overlapping suggests that the floor 
plates may be modified thecal plates belonging to the upper face 
of the theca. This would make the floor plates of the Agela- 
crinidae epithecal, the food grooves extending over the thecal 
plates themselves, without intermediate flooring. In this respect 
they are similar to the Diploporita among the Cystidea, from which 
they differ in other important particulars. 
