53 
latter corresponding to the most prominent part of the several 
ridges. 
The secondary spines are much more simple in structure, 
no external dense crust being found on them, the pith having 
the longitudinal fibres very strongly developed, and these 
fibres passing gradually towards the surface into anill defined 
jayer, which corresponds with the middle layer of the primary 
spines. 
The articular surfaces of both descriptions of spines, 
together with the tubercles on which they articulate, are con- 
solidated by the deposition of calcareous matter between the 
reticulate fibres which primitively compose them. This con- 
dition of greater density is probably brought about by the 
friction to which these parts are exposed. 
Pedicellarie (figs. 2, 3, 4, 5).—The structure of the Pedi- 
cellariz is peculiar to the Cidaride. They are commonly of 
but one essential form, modified only to a slight extent in the 
various genera, and in the different parts of the shell of the 
same species. They consist of a head formed of three jaw-like 
pieces supported on a stem; the size of the head and the 
length of the stem vary considerably in the different parts of 
the shell. Each spoon-like jaw is in contact throughout the 
entire length of its serrated edge with both its fellows. They 
differ from those of all other Echinoidea in having a large 
chamber in their outer wall occupying a third of the diameter 
of the jaw, and extending for about two thirds of its length 
from the apex ; it is closed at the end nearest the stem, but 
has a large triangular opening near the point, the margins of 
which are thickened and armed with strong teeth. The jaws 
of the Pedicellariz articulate at their base, as in the other 
Echinoidea, by a strong, broad, triangular and grooved pro- 
cess, which projects inwards and slightly forwards ; from this 
articular process arises a flat triangular keel, having its outer 
edge attached to the mesial line of the jaw to nearly its 
middle. The muscles which close the jaws arise and are in- 
serted on either side of the keel, where it is attached ex- 
ternally ; their combined action can readily be seen by referring 
to fig. 5. The muscles which open the jaws arise from that 
part of the articular process and base of the head which faces 
the stem; they run in part from jaw to jaw, and in part are 
inserted into the stem. The stem also presents peculiarities 
confined to this family, It is composed of two calcareous rods 
continuous with one another ; the upper one is very short and 
smooth, and is in close contact with the base of the head, to 
which it is united by the opening muscles of the jaws; the 
lower one composes the greater part of the stem, is twice the 
