ECHINUS. 307 
The great bulk of the pyramid consists of five pairs of 
jaws, each pair forming an alveolus (PI. L., figs. 9, 10, and 
12, alv.). Each alveolus is a hollow, triangular pyramid 
(fig. 11), two of the surfaces of which are lateral (figs. 
9, 11, and 12), and applied to the corresponding surfaces 
of the alveoli on either side, while the third is outer and 
| slightly rounded (fig. 10). The suture, which marks the » 
line of union of the pair of jaws, runs down the centre of 
the outer surface (swt.). The lateral surfaces are traversed 
by fine transverse and slightly sinuous grooves (figs. 9 
and 12), and the inner edges of these surfaces, which 
approach each other closely only at the base of the alveolus, 
are finely toothed. Hach alveolus lodges a long, slender, 
and slightly curved tooth (figs. 9, 10, and 12, ¢o.), the 
hard, pointed tip of which projects beyond its apex. The 
shaft of the tooth is convex towards, and closely applied 
to, the inner surface of the rounded outer side of the 
alveolus (fig. 11), exactly in line with the median suture. 
Tits upper basal end projects some distance beyond the 
base of the alveolus, and curves upon itself in the direction 
of the axis of the lantern. A prominent longitudinal 
ridge or carina traverses the inner concave surface of the 
tooth from near its pointed tip to a point corresponding 
with the base of the alveolus. Here the substance of the 
tooth becomes soft and readily yields to the pressure of 
the fingers in the form of asbestos-like fibres. Here, too, 
growth of the tooth takes place. Five oblong intermediate 
plates, the rotule (Pl. I., figs. 8 and 12, rot.), rest upon 
the bases of the alveoli, in such a way as to touch the 
apposed lateral surfaces of every two, while five slender, 
curved rods, the radii (Pl. I., fig. 12; Pl. IIL, fig. 23, rad.), 
lie, one upon each of them. The peripheral ends of the 
radii are bifurcate, and project over the edge of the 
- pyramid formed by the five alveoli. The alveoli are inter- 
