EErORT ON THE CRINOIDEA. 7 



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 Bourgueticrinidse 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 fossae lodge the elastic iuter-artieular 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 Vila. fig. 1, Is). 



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 generall)'. 

 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 Bourgueticrinidae this is by no means the case, for 

 the long axes of the terminal faces cross one another at various angles (PL Villa, 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 foss33 are shallowest (PI. VII. figs. 12, 13; PL Vila, 

 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 Vila. 

 fig. 1) become relatively slxorter 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 filjres are continuous through their whole length 

 (PL Vllb.), as is the case in the upper stem-joints of Pentacrinus (PL LVIII. 

 fig. 3, Is). 



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 M^stricht 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 Bourgueticrinidae 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 



i Encriiiiden, Tab. 104, fig. 70. 



