REPORT ON THE CRINOIDEA. cf 
ligamentous bundles, muscles being confined exclusively to the rays and their sub- 
divisions. 
This ligamentous articulation of the successive cirrus-joints and of the stem-joints of 
the Bourgueticrinidz is of a very simple nature. There is an articular surface around 
the opening of the central canal with a more or less marked fossa on either side of it 
(Pl. VIla. figs. 7-11). The fosse lodge the elastic inter-articular ligaments, which are of 
precisely the same nature as the tendinous cords in the stem of Pentacrinus; but they 
differ from these cords in being confined to the intervals between the elongated joints 
without traversing their substance (Pl. VIIa. fig. 1, 7s). 
The two ligaments may be of unequal size, as in the middle and outer portions of the 
cirri, whereby the prehensile power of these organs is much increased ; or they may be of 
equal bulk, as in the lower portions of the cirri and between the stem-joints generally. 
In the former, however, the long axes of the terminal faces of any joint lie in the same 
plane ; but in the stem-joints of the Bourgueticrinide this is by no means the case, for 
the long axes of the terminal faces cross one another at various angles (PI. VIIa. figs. 2, 3 ; 
Pl. IX. fig. 3). The outline of the interarticular ligaments of the middle and lower 
parts of the stem, as seen in optical or in longitudinal section, has a diamond shape with 
somewhat extended lateral angles. This is, of course, due to the fact that the fibres are 
longest in the centre of the stem, immediately around the vascular axis, and shortest 
towards the periphery where the fossze are shallowest (Pl. VII. figs. 12, 13; Pl. VIIa. 
figs. 7-11; Pl. X. figs. 11-14). 
But there are no fibres at all in the middle half of each joint, which is composed of 
the usual limestone network. Towards the upper part, however, the fibres (Pl. VIIa. 
fig. 1) become relatively shorter in the centre but longer and longer towards the 
periphery, and their ends approach more and more nearly to the middle planes of the two 
joints which they unite. By the time the joints become cylindrical and then discoidal 
immediately below the cup, the fibres are continuous through their whole length 
(Pl. VIIb.), as is the case in the upper stem-joints of Pentacrinus (Pl. LVIIL. 
fig. 3, Zs). 
The articular surface which surrounds the opening of the central canal varies 
considerably in its character. In many cirrus-joints it is merely an expansion of the 
thickened rim of the opening, but does not extend across the joint face. Quenstedt 
figures a stem-joint of this kind from the Mestricht chalk.! It is nominally referred to 
Bourgueticrinus ellipticus, but I have seen no recent ones like it. In other cirrus-joints 
and in the stem-joints of the Bourgueticrinide the articular surface takes the form of a 
more or less well-marked ridge, which lies either across or in the direction of the long 
axis of the oval joint-face, and is pierced in the middle by the opening of the central 
1 Eneriniden, Tab. 104, fig. 70. 
