ECHINODERMATA OF THE BRASSFIELD FORMATION 
29 
that in each case the slope is from the upper, distal margin to- 
ward the lower, proximal margin of the plate, forming an angle 
of about 15 degrees with the horizontal plane of the specimen. 
The surfaces of contact between these plates are finely striated 
in a direction perpendicular to their inner margins, similar to 
the striations on the inner surfaces of contact on some of the 
larger scales of some of the Ordovician Agelacrinidae. In the 
specimen here described these plates evidently were closely ar- 
ticulated, and probably were held rigidly together, or were only 
slightly movable. 
In the interbrachial arc between rays I and II, and also be- 
tween rays IV and V, there are several plates of which those 
near rays I and V are shorter, while those near rays II and IV 
are longer, in a radial direction. When viewed from the interior 
of the disk, these plates or series of plates tend to slope in the 
same direction as those between rays I and V, already described. 
Large, flat, scale-like plates are present also between rays II 
and III, and between rays III and IV. These also are finely 
striated in a direction from their distal toward their proximal 
margins, along their surfaces of contact. They slope somewhat 
as the interbrachial plates between rays I and V, rising laterally 
toward rays II and IV, and inclined more or less downward to- 
ward ray III. 
Mr. Austin H. Clark, of the United States National Museum, 
kindly showed the writer various Ophiuroidea with more or less 
overlapping scale-like plates, but in these Ophiuroidea the sym- 
metry was plainly radial, while in the specimen here described 
the symmetry seems more bilateral, as far as the arrangement 
of the scales of the disk are concerned. Unfortunately the upper 
surface of the disk is not visible. . On the basis of at least partial 
bilateral symmetry, the interbrachial arc between rays I and V 
is regarded as the posterior part of the disk. 
The outlines of the inner surfaces of the abactinal plates of’ 
the rays are exposed best in case of ray IV. As here exposed, 
the plates are irregular in size, form, and arrangement; there 
appears to be no arrangement in longitudinal series, correspond- 
ing to the radials, marginals, and inframarginals among the 
