oi 
that they are capable of resisting the action of the waves 
for a considerable time. The mouth is armed with jaws, 
inserted in five triangular spongy bones, and the pores 
are placed in five arched pairs of bands, forming a star 
on the upper part of the shells. Some of the species of 
this group, as the Clypeasters (Case 2) are convex and 
shield-shaped ; the others, as the Echinodisci and Scu- 
tellse, (Cases 2 and 3,) are so flat and depressed, that it is 
difficult to conceive how the animal can exist in so narrow 
a cavity. Many of the species of this genus are pierced 
with holes through the disk, ( Scutella quinquifora , 
Scutella bifora ), &c. Others are lobed on the margin, 
as the Eight-rayed Scutella ( Scutella octodactyla ) and 
Toothed Scutella {Scutella dentata ). 
In the third group, the bodies are orbicular, more 
or less depressed, with the two openings of the ali¬ 
mentary canal placed opposite each other in the axis 
of the shell, one at the vertex, the other at the base, 
and the series of pores forming bands, extending 
from the one to the other. These shells are generally 
covered with larger spines and tubercles. In many 
species of this division (Cases 3—6), the spines are 
of nearly equal size, and the tubercles on which they 
are placed not pitted in the centre ( Echinus); as the 
common Sea-Egg of the English coast (. Echinus escu - 
lentus), much sought after as food during a part of the 
summer season, at which time the shell is almost entirely 
filled with eggs. Other species, in which the tubercles 
are of the same form, have some spines much longer 
than the rest ( Echinometra ); as the Spiniferous Sea- 
Egg {Echinus lucunter ), the Triangular-spined Sea- 
Egg ( Echinus trigonarius) i and the Artichoke, or 
Black Sea-Egg ( Echinus atratus) f peculiar for the 
larger spines being very short and truncated, forming 
a smooth surface, somewhat resembling a tessellated 
pavement. In some species with spines of unequal 
size, the tubercles to which they are attached are 
pierced in the centre ( Cidaris ). Those called Tur- 
e 2 bans 
ROOM X. 
Nat. Hist. 
