res. In all the modern
Sea-urchins each of these ten zones, whether perforate or
imperforate, is composed of two rows of plates; and there are
thus twenty rows of plates in all. In the Palaeozoic Sea-urchins,
on the other hand, the "ambulacral areas" are often like those of
recent forms, in consisting of _two_ rows of perforated plates
(fig. 119); but the "inter-ambulacral areas" are always quite
peculiar in consisting each of three, four, five, or more rows
of large imperforate plates, whilst there are sometimes four
or ten rows of plates in the "ambulacral areas" also: so that
there are many more than twenty rows of plates in the entire
shell. 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. (After Dawson.)]

Amongst the _Annelides_ it is only necessary to notice the little
spiral tubes of _Spirorbis Carbonarius_ (fig. 120), which are
commonly found attached to the leaves or stems of the Coal-plants.
This fact shows that though the modern species of _Spirorbis_
are inhabitants of the sea, these old representatives of the
genus must have been capable of living in the brackish waters
of lagoons and estuaries.

[Illustration: Fig. 121.--_Pr