430 
Aug. F. Foerste 
The original form of the theca was circular, but in its present 
condition it is somewhat distorted, one of the diameters being 25 
mm., while the other, in a transverse direction, is about 20 mm. 
in length. The longest ray, measuring along the curvature, is 
20 mm. in length. All of the rays are so strongly curved that the 
interambulacral areas are narrow even in the central parts of the 
theca. The posterior area is about 3 mm. in width. All of the 
other interambulacral areas are about 2 mm. in width. 
At the proximal ends of the ambulacral rays, the floor plates 
enlarge laterally and also downward (plate I, Fig. 7A; also plate 
IV, Fig. 2), so as to form a marginal rim enclosing a central cavity, 
directly beneath the mouth. This part is well exposed but has 
been weathered so that the details can not be determined except- 
ing for a few plates. The middle plate along the anterior part 
of the rim is formed by the modified proximal floor plate of the 
anterior ray. This plate is concave toward the cavity, and its 
lateral edges overlap the edges of the adjoining marginal plates, 
on the right and left parts of the anterior rim. Towards its 
anterior extremity, this plate narrows down to the same width 
and has the same transverse curvature as the other floor plates. 
The left part of the anterior margin of the rim, as seen from below 
is formed by a similar modification of the proximal floor plate of 
the right ray (No. 4), but the overlapping sides of this plate do 
not extend as far on the posterior side as on the anterior one. 
Posterior to the latter is the modified proximal floor plate belong- 
ing to the right posterior ray (No. 5). 
On the right side of the anterior part of the rim of the substomial 
cavity, as seen from below, there is a modified proximal floor 
plate belonging to the left ray (No. 2). Immediately posterior 
to this should be the modified proximal floor plate belonging to the 
left posterior ray (No. 1), but the latter is badly weathered, and 
only the narrow distal termination of this plate can be identified 
with confidence. The posterior outline of the cavity is formed a 
mass of material in which it has not been possible to recognize 
any definite structure. 
Immediately in front of the median part of the posterior part of 
the rim there is a vertical cavity, less than a millimeter in diam- 
eter, apparently leading to the oral surface of the theca. This 
aperture, as seen from below, is bounded on the left by a 
quadrangular plate filling in most of the lower left hand quarter 
