6 



between the joints are widest on the side nearest the 

 vertical axis of the animal, towards which the whole 

 cirrus is always more or less flexed. 



Radial Plates.— These plates (PL V., fig. 52; PI. VI., 

 figs. 56 and 59, rd.), closely adhering to each other by 

 their lateral faces and together forming a well marked 

 pentagon, are also adherent by their aboral faces 

 (figs. 52 and 59) to the oral face of the centro-dorsal plate. 

 Each radial plate is of triangvilar form. Its slightly 

 truncated apex, forming the internal face of the plate 

 (PI. I., fig. 4), is turned towards the vertical axis of the 

 animal, its base forms one of the sides of the radial 

 pentagon, while its lateral faces are in close contact with 

 its fellows on either side. The lateral faces are flat, and 

 each presents a fairly large aperture in the angle 

 formed by the aboral and internal faces (fig. G). Two 

 apertures of slightly larger size appear in the latter face, 

 which is small and irregular (fig. 4). The external 

 face (fig 5) presents an oval aperture in the middle 

 of its width, and a well marked transverse ridge divides 

 this face into upper and lower portions. The former is 

 divided into two pairs of fossse by an incomplete transverse 

 ridge and the continuation of two converging ridges, which 

 are more conspicuous on the oral face of the plate (fig. 7j. 

 The upper, deeper pair lodge the proximal ends of the 

 powerful flexor muscles, while the lower, shallow pair 

 aft'ord attachment for the proximal ends of the inter- 

 articular ligaments. The lower (aboral) portion of this 

 face is occupied by a single fossa which lodges the 

 proximal ends of an elastic extensor ligament. The 

 oral face of the plate presents two curved ridges (fig. 7), 

 the shorter ends of which approach each other along 

 the median line and form the median edges of the pair 

 of fossae which lodge the flexor muscles. The apertures 



