118 



Description. 



Dorsal cup forms a wide cone with a rather broad base (figs. 160, 179). 



I BB 5, smooth not protuberant. 



BB 5, hexagonal, cxcept the post. and r. post. which are heptagonal; the side by 

 which they raeet is, however, an exceedingly short one. 



KR 5, branching ont from the cup in a line with the arms. They are often slightly 

 constricted below the facet, which occupies the greater part of their width. 



R' has straight sides and is fairly large for the genus. 



Arms: IBr 2, very rarely 1, in each ray; about half as wide again as high. As 

 a rule they are slightly constricted in the distal half, then swell ont above in a slight ridge. 



The two main branches of the arms give off armlets on alternate sides from everv 

 first, second, third or fourth ossicle. These armlets sometimes extend almost as far as 

 the main arms themselves; they give off branches on alternate sides, which themselvea 

 ramify. The main or distichal arms are heavy, the secundibrachs being almost as wide 

 as the primibrachs. The tetrastichal armlets are about two-thirds the size of the distichal 

 arms or less, while the branches which they bear are slender and short. All the brachials 

 are slightly constricted in the same way as the primibrachs, and are of about the same 

 proportions. 



In section the arm-ossicles are sub-quadrangular or almost circular, with the ventral 

 side excavated by a broad shallow groove (fig. 172). The ax i al canal is slightly dorsad 

 of the centre of the ossicle, and traces of its origin are seen in a fissure that connects it 

 with the ventral groove, which fissure is as a rule closed and parti y obscured by stereom. 

 The axial canal lies in the middle of a slight transverse ridge, dorsad of which is a very 

 faint depression of semicircular outline; while ven träd of it, and on either side of the 

 fissure, is a small but well-marked triangulär depression. This description is taken from 

 a large specimen; in smaller specimens the muscular and ligamentous depressions are less 

 marked, while the fissure is more open (compare Ann. Mag. Nat. Hist., ser. 6, vol. VII, 

 Pl. XIII, fig. 10). 



The covering-plates are normally of pentagonal outline and alternate with one an- 

 other (fig. 161). In older specimens, however, the arms are huge and niassive, and are 

 very like those of Onychocrinus; in them the ventral groove is covered by a number of 

 irregular plates of varying size like those that form the tegmen [see Text-tig. 17 (4), and 

 my previous Botryocrinus paper, (op. rit. 1891, p. 406)]. Another arrangement is shown 

 in figs. 171 — 1 7 ; > : the covering-plates are in two series, viz., a median series of smaller 

 alternating plates, and an outer, discontinuous series of larger, irregular plates which lie 

 över the interbrachia! sutures; the median series rests on the bevelled edges of the outer 

 plates as well as on the edges of the ventral groove. 



Anal struetures: x is of much the same shape as the radials, and is, like them. 

 slightly constricted about one third of the way down. Its lower sides, however, are not 

 (piite equal, that abutting on W being rather smaller than that abutting on post. B. 



The Ventral Sac is composed of hexagonal plates. In the proximal part of the sac 

 these plates are arranged in regular longitudinal series, one of which ean be traced right 



