A few Star-fishes and Brittle-stars are known to occur in the Carboniferous rocks; but the only other Echinodemls of this period which need be noticed are the Sea-urchins (Echinoids).  Detached plates and spines of these are far from rare in the Carboniferous deposits; but anything like perfect specimens are exceedingly scarce.  The Carboniferous Sea-urchins agree with those of the present day in having the body enclosed in a shell formed by an enormous number of calcareous plates articulated together.  The shell may be regarded as, typically, nearly spherical in shape, with the mouth in the centre of the base, and the excretory opening or vent at its summit.  In both the ancient forms and the recent ones, the plates of the shell are arranged in ten zones which generally radiate from the summit to the centre of the base.  In five of these zones—­termed the “ambulacral areas”—­the plates are perforated by minute apertures or “pores,” through which the animal can protrude the little water-tubes ("tube-feet”) by which its locomotion is carried on.  In the other five zones—­the so-called “inter-ambulacral areas”—­the plates are of larger size, and are not perforated by any apertures.  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.