408 Proceedings of the Royal Society 
radials of all other recent Crinoids in being connected with the 
radial axillaries by a true muscular joint instead of by a syzygy. 
The alternative is that they may be the upper articulating surfaces 
of the first radials, in which case the next joints may be formed 
of the second and third radials coalesced, and the syzygy between 
them obliterated ; or, finally, there may be only two radials. There 
is no trace of any further division of the wall of the column, and 
the cavity is continued contracting gradually to the bottom. A 
vertical mark, sometimes a groove and sometimes a ridge, runs 
from the centre of each articulating facet down the inside of the 
wall for about two-thirds of the depth of the cavity, when it is lost. 
The upper border of the cup, bearing the facets, is very irregular 
in thickness ; and in all the specimens which I have seen, including 
D’Orbigny’s, one side of the border is much thicker and consider- 
ably higher than the other side, and the three arms articulated to 
it are much larger than those articulated to the opposite side. 
There is thus a very marked division into “ bivium ” and “ trivium,” 
and consequently a bilateral symmetry underliestheradiated arrange- 
ment of the antimeres. Singularly enough, the specimen described 
by D’Orbigny was abnormal, only four arms being developed, a 
circumstance which no doubt greatly conduced to the doubt with 
which its determination as an echinoderm was received. 
Five “ radial-axillary ” plates, three larger and two smaller, 
articulate by corresponding ridges and muscular impressions with 
the facets of the border of the cup, and each of these bears dis- 
tally two facets, sloping outwards and downwards, for the insertion 
of the first brachials. The outer surfaces of the radial axillaries are 
very gibbous, thrown out into almost hemispherical projections, 
studded with low tubercles ; a deep groove runs up the centre of 
the inner aspect of the joint, and the two sides send inwards very 
strong projecting processes, which abut against the corresponding 
processes of the contiguous joints on either side, and lock into them 
by a system of corresponding ridges and grooves, so that there 
appears to be little or no motion between these joints. 
The radial-axillaries are each succeeded by two series of about 
eight similar, thick, wedge-shaped brachial joints, very convex 
externally, and giving off laterally on each side of the arm alter- 
nating, very flat broad pinnules each consisting of about six plate- 
