Agelacrinidae and Lepadocystinae 
403 
efforts to escape contamination by excreta. In this case it is 
noted that the predominating position of the animal is such as to 
bring the proximal parts of the left (No. 2) and right (No. 4) rays, 
and the connecting peristomial slit, into parallism with the direc- 
tion of the prevailing currents^ as determined from the slope of the 
Rafinesquina upon which the animal rested. By the peristomial 
slit is meant the slit formed by the peristomial plates^ which 
extends from the median line of the left ray to the median line of 
the right ray (or from plate Z to plate Y in Figure 5B, on plate I) . 
In this orientation, the proximal part of the right ray is directed up 
the slope. This position of the animal probably accounts for the 
direction of curvature of the rays in Agelacrinus pileus. 
4. Curvature of Ambulacral Rays 
Some of the early forms of Agelacrinus, such as Agelacrinus 
hillingsi, Chapman, from the Trenton, and Agelacrinus hohemicus, 
Barrande, from Etage D, have the rays sharp and quite straight, 
abutting against or tapering to a broad peripheral margin of larger 
and smaller plates. In another Trenton form Agelacrinus dick- 
soni, Billings, all the rays are strongly curved in a contrasolar 
direction. This may be regarded as the primitive condition 
among species with curved rays. The primitive contrasolar curva- 
ture of the rays is indicated even by those Ordovician species in 
which the right ray is strongly curved in a solar direction distally, 
since, at the proximal end, this ray begins with a contrasolar curva- 
ture. This contrasolar curvature of the proximal part of the right 
(4) ray is well shown by both Agelacrinus pileus (plate II, Figs. 1, 2) 
and Agelacrinus cincinnatiensis , and probably was shown also by 
other Ordovician species of which this part of the theca, at present, 
is unknown. 
The curvature of the rays, whether in a solar or contrasolar 
direction, was due to the extensionof the distal part of the rays in 
the effort to secure more food. This extension could not proceed 
beyond the inner part of the peripheral ring since this was formed 
by quite large plates, probably held together fairly rigidly. The 
extension of the rays, of necessity, therefore, took place along the 
inner margin of this ring. What caused it to start in a contrasolar 
direction is unknown, but the fact is evident. 
The development of species with strongly curved rays from those 
