Some of the Palaeozoic Sea-urchins, also, exhibit a very
peculiar singularity of structure which is only known to exist
in a very few recently-discovered modern forms (viz., _Calveria_
and _Phormosoma_). The plates of the inter-ambulacral areas,
namely, overlap one another in an imbricating manner, so as to
communicate a certain amount of flexibility to the shell; whereas
in the ordinary living forms these plates are firmly articulated
together by their edges, and the shell forms a rigid immovable
box. The Carboniferous Sea-urchins which exhibit this extraordinary
peculiarity belong to the genera _Lepidechinus_ and _Lepidesthes_,
and it seems tolerably certain that a similar flexibility of
the shell existed to a less degree in the much more abundant
genus _Archoeocidaris_. The Carboniferous Sea-urchins, like the
modern ones, possessed movable spines of greater or less length,
articulated to the exterior of the shell; and these structures
are of very common occurrence in a detached condition. The most
abundant genera are _Archoeocidaris_ and _Paloechinus_; but the
characteristic American forms belong principally to _Melonites,
Oligoporus_, and _Lepidechinus_.
[Illustration: Fig. 120.--_Spirorbis (Microconchus) Carbonarius_,
of the natural size, attached to a fossil plant, and magnified.
Carboniferous Britain and North America.
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