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at a certain distance from the centre, consolidate into a rib 
or pillar, which runs from the root to the apex of the spine, 
forming a longitudinally furrowed exterior, caused by the 
centre of each rib projecting slightly. Once this consolida- 
tion of tubes into ribs effected, the growth changes, the ribs 
extending eccentrically into plates radiating from the centre, 
and separated from each other by a mass of tissue similar to 
that at the centre. At the close of the second period of 
growth these plates thicken concentrically so as almost to 
touch each other, which however they do not, leaving a 
furrow of separation. They then continue their radiating 
extension till the end of another year’s growth, when the 
concentric thickening into an outer rib again takes place, 
and so on in successive years. These radiating plates are 
horizontally perforated by circular apertures bearing a 
singular resemblance to rivet holes in boiler plates, and 
appear to have rivets passing through them of the opaque 
substance in a fibrous state. Altogether the structure of 
the spine may be compared to that of an iron tubular bridge. 
The thickened exteriors of these plates are highly coloured, 
and traversed by winding, anastomising channels containing 
air; in some portions of the spine, the opaque substance 
either grows into, or is destroyed and replaced by, the trans- 
parent substance, which then forms a solid mass, perforated 
by the rivet holes, now changed into winding, anastomising 
passages. 
Forms intermediate between the extremes of complexity 
and simplicity were those of Echinus sphsera and lividus. 
Echinus sphsera being chiefly composed of the pith-like 
substance, with 25 or 30 radiating glassy plates of a whitish 
