202 THE VOYAGE OF H.M.S. CHALLENGEE. 



dorsal" in the sense in which it is usually understood, i.e., as the cirrus-bearing top stem- 

 joint of the Comatulae. 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 "cu23ule." He speaks of the axillaries which are articulated to its upper edge as 

 " radiales uniques," resting as in Cyathidium, " sur les angles de la cu2iule, qui pourrait 

 done etre evisagee comme etant composee de cinq pieces basales interradiales." This 

 would be a most singular morphological condition, and one without a parallel in any 

 (ither Crinoid. Primary radials would be in contact wdth each other, but not united, and 

 rest on articular surfaces each of which would be formed by the upj)er 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 afibrded 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 Eudesicrimis. 



Sir AVyville Thomson thought it probable that second radials are also present in the 

 cup. If so, they must be imited 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 ease 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 difi"er 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 IlL fig. 1. Besides this again the individual 

 facets, both of bi\dum and trivium, are of difi"erent sizes and shapes. The articular ridge 

 which crosses the central facet of the trivium is considerablj' 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 Paleont. Frang., he. cit., p. 191. - Loc. at., pp. 407, 408. ^ Loc. cit., p. 408. 



