428 
Aug. F. Foerste 
At the center of the theca, as seen from below (plate I, Fig. 5A), 
there is a substomial chamber or cavity from which the ambulacral 
rays radiate. The anterior part of the rim of this cavity has a 
form suggesting a pentagonal outline for the entire rim. This is 
owing to the fact that this part of the rim appears to be formed by 
the enlargement and lateral extension of the proximal floor plate 
of each of the three anterior rays, usually called the left, anterior, 
and right rays, or rays Nos. 2, 3, and 4. Even if eventually the 
homology of the rim plates with the proximal floor plates be dis- 
proved, the appearance of the rim plates is very well described by 
this supposed homology. If the specimen be held with the oral 
side upward, then the proximal floor plate of each of these anterior 
rays appears to widen rapidly so as to overlap the proximal floor 
plate of the neighboring ray; at the same time this proximal floor 
plate curves downward more or less vertically, so as to produce the 
rim-like effect, as seen from below. The outline of this rim is 
sufficiently straightened between the median parts of the left and 
anterior rays, and between the anterior and right rays, to suggest 
the pentagonal outline. As seen from below, each of the three 
proximal floor plates arches over the ambulacral groove to which 
it belongs, leaving an oval opening leading from the median 
groove in the ambulacral rays into the central substomial cavity. 
It must be evident, however, in view of the long transverse 
peristomial slit, as seen from the exterior view of the specimen, that 
a corresponding transverse elongation of the substomial chamber 
must be expected from an interior view, and this is the case. The 
regularity of the supposed pentagonal outline is considerably dis- 
turbed by the acuteness of the angle at which the proximal floor 
plates of the posterior rays meet those of the lateral rays. This 
feature is best shown by specimens No. 13266-1-r and x, belong- 
ing to the American Museum of Natural History. In both of 
these specimens, the proximal floor plate of the right posterior ray 
(No. 5) meets that of the right ray (No. 3) at quite a considerable 
angle. It is assumed that a similar angulation existed at the 
angle between the left and left posterior rays. (Plate II, Fig. 3.) 
The posterior margin of the substomial chamber, in the Miller 
and Faber specimen, is formed by a large quadrangular plate, 
which appears to be merely the lower part of the large quadran- 
gular peristomial plate seen on the posterior side of the peri- 
stomial slit as viewed from the exterior. On its inner face, within 
