202 THE VOYAGE OF H.M.S. CHALLENGER. 
dorsal” in the sense in which it is usually understood, 7.e., as the cirrus-bearing top stem- 
joint of the Comatule. The tubular body-chamber of Holopus is, however, distinctly 
not of this nature; so that the use of the name “‘ centro-dorsal ” is apt to lead to confusion. 
In a subsequent passage ' de Loriol expresses another view of the composition of this cup 
or “cupule.” He speaks of the axillaries which are articulated to its upper edge as 
“yadiales uniques,” resting as in Cyathidium, “sur les angles de la cupule, qui pourrait 
done étre evisagée comme étant composée de cing pitces basales interradiales.” This 
would be a most singular morphological condition, and one without a parallel in any 
other Crinoid. Primary radials would be in contact with each other, but not united, and 
rest on articular surfaces each of which would be formed by the upper edges of two 
basals. 
The union between basals and radials is invariably a simple synostosis such as I have 
described above (pp. 2, 3), and never a muscular joint like that between the upper edge 
of the calyx-tube of Holopus and the compound axillaries. The evidence afforded by 
sections of the cup, however, indicates clearly that it is principally composed of closely 
united first radials which, as will be pointed out subsequently, have a remarkable 
similarity to the radials of the Liassic genus Hudesicrinus. 
Sir Wyville Thomson thought it probable that second radials are also present in the 
cup. If so, they must be united to the first by synostosis, which would be a most 
unusual condition in any Neocrinoid; and the close resemblance of Holopus to Eudesi- 
crinus seems to negative this idea altogether ; while, as pointed out by Sir Wyville,’ there 
would be a true muscular joint between the second radials and the radial axillaries, which 
is not the case in any other recent Crinoid. 
Considering then the articular surfaces at the edge of the calyx tube as those of 
first radials, we find that they differ considerably in size. According to Sir Wyville 
Thomson ’ “‘ 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 considerably higher than the other side, and the three arms articu- 
lated 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 underlies the radiated arrangement of the antimeres.” This is shown in Pl. 
V. fig. 1, and also, though less clearly, in Pl. III. fig. 1. Besides this again the individual 
facets, both of bivium and trivium, are of different sizes and shapes. The articular ridge 
which crosses the central facet of the trivium is considerably longer than that of either of 
the two remaining facets, and these are longer than the ridges on both the bivial facets. 
The adjacent muscular plates of these two last are fused into a short tongue-shaped 
process which stands up prominently in the angle of the bivium. It is essentially of the 
same nature as the “clavicular piece” which projects in the middle of the distal 
1 Paléont. Frane., loc. cit., p. 191. 2 Loe. cit., pp. 407, 408. 3 Loc. ctt., p. 408. 
