174 Prof. M‘Intosh’s Notes from the 
a fissured and spinous rim of sixteen to twenty or more 
processes. Moreover, the upper surface of the disc is 
armed with spikes, which form a series of rings round 
the central area. In lateral view it is vase-shaped, with 
a graceful inclination outward and upward from the 
narrow stem. ach of the rays forms a stout tapering 
process, terminated by a thin flexible tip, each side beg 
armed with three or four stiffer and shorter spines, a double 
contour being visible for fully the basal half. The longest 
lateral spines are, as a rule, the distal pair, since they have 
a wider area to guard. This elegant horny apparatus takes 
the place of the calcareous operculum of other forms, and 
the flexible tips of the rays apparently yield to a certain 
extent in withdrawal into the tube, thus causing the spines to 
approach each other more closely, and efficiently protecting 
the aperture. In some large examples the dorsal and lateral 
spikes are bifid, various smaller spikes occur in the intervals, 
and an imperfect ray is intercalated between two of the 
others. 
The second tier of the operculum forms a graceful fluted 
cup with about twenty-seven crenations, each separated 
from its neighbour by an involution of the tough integu- 
ment enclosing hypodermic cells and granules. The mus- 
cular fibres in the centre of the stem expand as they 
approach the lower operculum, the centre of which they pull 
on, and so fix the elastic crenate margin on the sides of the 
tube. The protection of the aperture is thus doubly secure, 
for the distal spiny shield closes over the crenate cup and 
exposes only the tough spiny rays to an aggressor. It 
requires great force to detach this cup from the opercular 
stalk, apparently from the toughness of the cuticle. The 
rudimentary organ of the other dorsal edge is often minute, 
resembling a Loxosoma, and springs from the fan near the 
base of the first filament of the side. 
The dorsal edge of the collar, which is supported by the 
first bristle-bundles, forms the commencement of the alar 
membrane, which passes backward over the anterior bristles 
to their termination, beyond which its thin edge projects 
ventrally as a flap of a bluntly conical outline. In the 
preparations, as a rule, the anterior edges of the alar mem- 
brane approach each other, whilst the posterior margins, as 
they trend to the ventral surface, are wide apart. The 
bristle-tufts stand clear of the membrane on each side, and 
the free ventral flap is ample. The dorsal surface in this 
region is somewhat flattened, whilst the ventral is slightly 
convex and marked by a median groove. In the second 
