REPORT ON THE CRINOIDEA. 190 



Holopxis rangi, d'Orbigny (PLs. I.-Vb. ; PL Vc. figs. 1-3). 



Diraensions. 



Total height, .... 



Greatest height of cup on trivial side, 



Least height on bivial side, . 



Greatest diameter of upper edge of calyx, 



Greatest width of trivial axQlary (composite), 



Greatest width of arm. 



Diameter of smallest specimen (PI. V.), 



Height of smallest specimen, . 



A. (PL I.) B. (PL IV.) 

 40-00 mm. 8-50 mm. 



15-00 „ 4-25 „ 



5-25 „ 1-75 „ 



17-00 „ 5-00 „ 



13-75 ,, 3-75 „ 



9-75 „ 2-00 „ 



3-25 „ 



1-00 „ 



encnistiug calcareous expansion 



The tubular calyx which is attached by an irregular 

 of variable extent, is thick walled, inversely conical, and slightly bent to one side 

 (Pis. I., II., IV. ; PI. III. fig. 1). A more or less distinctly marked constriction separates 

 the spreading base from the actual cup, the greatest height of which, measured on the 

 convex side, is 15 mm. Its cavity narrows very rapidly from above downwards, so that 

 the thickness of its walls, which is everjTvhere considerable, is greatest at its lower 

 extremity (PI. V. figs. 1-4) ; and it is probable that the cup is completely closed below 

 by the spreading base, if not some little way above it. 



The analogy of other Crinoids leads one to believe that the cup is composed of radial 

 plates above, and of basals below ; but it is difl&cult to define the limits of either. The 

 radials, however, may be traced downwards some little way, owing to the difi"erences of 

 texture in the limestone network. Sections were made for Sir Wyville Thomson of the 

 least perfect of Sir Rawson Rawson's specimens. The articular faces round the upper edge 

 of the calyx are shown in PI. V. fig. 1 (compare also PI. III. figs. 1, 2). They were 

 described as follows by Sir Wjrv'ille Thomson ^ — " Each facet is traversed by a transverse 

 articulating ridge, a little in front of which there is the mouth of the tube which lodges 

 the sarcode axis of the joints, and a little behind its centre there is a somewhat longer 

 aperture which appears to lead into the cancellated structure of the outer part of the 

 wall. There are two large shallow muscular impressions on the surface of the facet on 

 the proximal aspect of the transverse ridge." The larger of these two apertures is not 

 the opening of a canal, like the smaller and inner one ; but it leads into a deep pit which 

 lodges the dorsal ligament connecting the radials with the joints above them. It 

 reappears upon the proximal faces of these joints, and upon the articular surfaces of all 

 the arm-joints (PI. III. figs. 3-15). In most Crinoids this pit is merely the deepest part 

 of a large fossa lodging the dorsal hgament (PI. Vila. fig. 15, Id'. PL Vllb, fig. 5 ; 

 PL Villa, fig. 7— Id ■ PL X. fig. 4 ; PL XX. fig 7) ; and there is an approach to this 

 condition in the later arm-joints of Holopus, which have a large portion of the articular 



1 Proc. Roy. Soc. Edin., 1876-77, p. 407. 



