50 IOWA STUDIES IN NATURAL HISTORY 
chamber,’’ of Holopus was formed of the basals, radials, and 
probably also the primary brachials fused together. He noticed 
that the upper portion of the hollow column expands slightly, 
and its thickened upper border is divided into five well de- 
veloped facets for the articulation of the five arm pairs, each 
facet bearing an axillary followed by two arms. These facets, 
he concluded, represent the upper surfaces of the primibrachs; 
but, if so, they differ from the primibrachs of all other recent 
erinoids in being united with the axillaries by a true muscular 
articulation instead of by an articulation of the non-muscular 
type. He suggests the alternative that they may be the distal 
articulating surfaces of the radials, in which case the following 
segments may be formed of the two primibrachial ossicles eoal- 
esced, and the non-muscular articulation between them obliter- 
ated; or, he says, there may be only one radial and one primi- 
brach ossicle. He describes the upper border of the cup bear- 
ing the facets as being very irregular in thickness, and in all 
the specimens which he examined, including d’Orbigny’s, one 
side of the border is much thicker, and consequently higher, 
than the other, and the three arm pairs articulated to it are 
much larger than those articulated to the opposite side. There 
is a very marked division into bivium and trivium, and con- 
sequently a bilateral symmetry underlies the radial arrange- 
ment. The axillaries are each succeeded by two series of about 
eight similar thick wedge-shaped brachials, very convex exter- 
nally, and giving off laterally, alternating on either side of the 
arm, very broad flat pinnules each consisting of about six plate- 
like segments. The brachials are also provided with strong 
lateral processes forming a wall on either side of the radial 
groove, and the sides of adjacent series of these first eight bra- 
chials are marked with corresponding grooves and ridges, s 
that, although from the presence of articulating ridges of vary- 
ing degrees of obliquity and of muscular impressions the proxi- 
mal portions of the arms must be capable of some motion, that 
motion would appear to be slight. After about the eighth the 
brachials suddenly contract in size and become greatly com- 
pressed, and this narrow series extends to about sixteen in num- 
ber, gradually tapering to the end of the arm. The facets on 
the edge of the hollow column he describes as follows: 
