Agelacrinidae and Lepadocystinae 
425 
In Thresherodiscus it has been impossible to determine whether 
the anus is located in the bottom of the depression on the right 
side of the posterior interambulacral area, or at the top of the 
immediately adjacent part of the elevation which is present on the 
distal side of this depression. If the latter was its location, then 
the anal pyramid consisted of small imbricating plates, not pre- 
senting a strongly radial arrangement. If the anus was located 
at the base of the depression, as more probably was the case, then 
nothing is known about the anal pyramid. 
In Cystaster granulatus the anal pyramid is abruptly elevated 
near the middle of the posterior interambulacral area. It is 
formed of about 10 somewhat elongated plates, more or less 
overlapping laterally, so that part are exterior and part more or 
less interior. 
24. Location of the Anus in the Agelacrinidae 
Since the Agelacrinidae may be regarded as derivatives from a 
Cystidean type, it may be worth while to emphasize the location of 
the anus in the posterior interambulacral area, opposite the 
anterior ray. This is a frequent position of the anus among the 
Diploporita, in Cystidea. While a similar position of the anus is 
described by Bather {The Echinoderma, 1900, p. 53) in case of 
Echinosphaera aurantium, from the Ordovician of Europe, the 
anus in most Ehomhifera has travelled toward the right, occurring 
between the right and right posterior branch of the ambulacral 
system in most Glyptocystidae, and between the anterior and right 
branches in the highly specialized genus Cystohlastus, from the 
Ordovician of Russia. 
25. Origin of Ambulacral System of the Agelacrinidae 
The fact that the lateral margins of the floor plates of the 
Ordovician Agelacrinidae passes beneath the adjacent margins of 
the interambulacral plates is opposed to their origin as exothecal 
p’ates, as in the Rhomhifera among the Cystidea. The fact that 
in Thresherodiscus, the terminal parts of the branching rays rest 
upon the margins of ordinary thecal plates, merely depressing their 
exposed margins a little, suggests the origin of the floor plates as 
parts of the original thecal covering. The latter structure is 
