410 
Aug. F. Foerste 
8825 of the Faber Collection, in Walker Museum, at Chicago 
University, preserves traces of thin plates belonging to the aboral 
surface, but I have interpreted these plates as displaced floor 
plates of the ambulacral rays. Specimens of Agelacrinus cincin- 
natiensis, in which a few of the plates of the oral side of the theca 
are missing, are not uncommon. On etching away, with caustic 
potash, the clay filling between this oral face of the theca and the 
surface of the Rafinesquina upon which it rested, no trace of aboral 
plates were discovered. The finest transverse lines, on the radiat- 
ing striae of the Rafinesquina^ however, were preserved. While 
this evidence is only negative, it may be assumed that in those 
forms which assumed the sessile habit, the original plates on the 
aboral surface became obsolete, a fleshy surface, unprotected 
beneath, being much better adapted for attachment to an under- 
lying surface. 
14. Interambulacral Plates 
In all of the Ordovician species referred to Agelacrinus or 
Lepidodiscus , the interambulacral plates are scale-like and more or 
less imbricating, overlapping each other in a proximal direction. 
The degree of overlapping of some of the plates may be small but 
nevertheless is distinct. It is always greater toward the periph- 
eral band. Even in Agelacrinus holbrooki (plate IV, Fig. 1), 
which Clarke describes as showing mosaic plates in the interradii , 
while squamose and imbricating at the margin (New Agelacrinites, 
1901, p. 189), the interambulacral plates overlap proximally at 
(juite acute angles although not for long distances. This is best 
seen where the plates are more or less loosened but not displaced. 
The plates are arranged more or less in rows crossing each other 
diagonally. This diagonal arrangement continues into the adja- 
cent rows of plates belonging to the peripheral ring. 
In view of the theory that the Agelacrinidae represent deriva- 
tives from a Cystidean ancestry, the imbricating squamose form 
of plates can scarcely be considered as primitive. The plates of 
Cystideans are polygonal and form a mosaic, and it is from a 
polygonal, mosaic stock of plates that those of the Agelacrinidae 
may be supposed to have originated. 
The change to imbricating plates probably was due to the assum- 
ing of the sessile habit, together with the enormous shortening of 
the theca in a vertical direction. This caused the distal edge 
