Passing onto higher animals, we find that the class of the _Echinodermata_ is represented by examples of the Star-fishes (_Asteroidea_), the Sea-lilies (_Crinoidea_), and the peculiar extinct group of the Cystideans (_Cystoidea_), with one or two of the Brittle-stars (_Ophiuroidea_)--the Sea-urchins (_Echinoidea_) being still wanting. The Crinoids, though in some places extremely numerous, have not the varied development that they possess in the Upper Silurian, in connection with which their structure will be more fully spoken of. In the meanwhile, it is sufficient to note that many of the calcareous deposits of the Lower Silurian are strictly entitled to the name of "Crinoidal limestones," being composed in great part of the detached joints, and plates, and broken stems, of these beautiful but fragile organisms (see fig. 12). Allied to the Crinoids are the singular creatures which are known as _Cystideans_ (fig. 46). These are generally composed of a globular or ovate body (the "calyx"), supported upon a short stalk (the "column"), by which the organism was usually attached to some foreign body. The body was enclosed by closely-fitting calcareous plates, accurately jointed together; and the stem was made up of numerous distinct pieces or joints, flexibly united to each other by membrane. The chief distinction which strikes one in comparing the Cystideans with the Crinoids is, that the latter are always furnished, as will be subsequently seen, with a beautiful crown of branched and feathery appendages, springing from the summit of the calyx, and which are composed of innumerable calcareous plates or joints, and are known as the "arms." In the Cystideans, on the other hand, there are either no "arms" at all, or merely short, unbranched, rudimentary arms. The Cystideans are principally, and indeed nearly exclusively, Silurian fossils; and though occurring in the Upper Silurian in no small numbers, they are pre-eminently characteristic of the Llandeilo-Caradoc period of Lower Silurian time. They commenced their existence, so far as known, in the Upper Cambrian; and though examples are not absolutely unknown in later periods, they are pre-eminently characteristic of the earlier portion of the Palæozoic epoch. [Illustration: Fig. 47.--Lower Silurian Crustaceans. a, _Asaphus tyrannus_, Upper Llandeilo; b. _Ogygia Buchii_, Upper Llandeilo; c, _Trinucleus concentricus_, Caradoc; d, _Caryocaris Wrightii_, Arenig (Skiddaw Slates); e, _Beyrichia complicata_, natural size and enlarged, Upper Llandeilo and Caradoc; f, _Primitia strangulata_, Caradoc: g. Head-shield of _Calymene Blumenbachii_, var. _brevicapitata_, Caradoc; h, Head-shield of _Triarthrus Becki_ (Utica Slates), United States: i, Shield of _Leperditia Canadensis_, var. _Josephiana_, of the natural size, Trenton Limestone, Canada; j, The same, viewed from the front. (After Salter, M'Coy, Rupert Jones, and Dana.)]