As regards the _Invertebrate animals_ of the Trias, our knowledge is still principally derived from the calcareous beds which constitute the centre of the system (the Muschelkalk) on the continent of Europe, and from the St Cassain and Rhætic beds still higher in the series; whilst some of the Triassic strata of California and Nevada have likewise yielded numerous remains of marine Invertebrates. The _Protozoans_ are represented by _Foraminifera_ and _Sponges_, and the _Coelenterates_ by a small number of _Corals_; but these require no special notice. It may be mentioned, however, that the great Palæozoic group of the _Rugose_ corals has no known representative here, its place being taken by corals of Secondary type (such as _Montlivaltia, Synastoea_, &c.) The _Echinoderms_ are represented principally by _Crinoids_, the remains of which are extremely abundant in some of the limestones. The best-known species is the famous "Lily-Encrinite" (_Encrinus liliiformis_, fig. 142), which is characteristic of the Muschelkalk. In this beautiful species, the flower-like head is supported upon a rounded stem, the joints of which are elaborately articulated with one another; and the fringed arms are composed each of a double series of alternating calcareous pieces. The Palæozoic Urchins, with their supernumerary rows of plates, the Cystideans, and the Pentremites have finally disappeared; but both Star-fishes and Brittle-stars continue to be represented. One of the latter--namely, the _Aspidura loricata_ of Goldfuss (fig. 143)--is highly characteristic of the Muschelkalk. [Illustration: Fig. 142.--Head and upper part of the column of _Encrinus liliiformis_. The lower figure shows the articulating surface of one of the joints of the column. Muschelkalk, Germany.] [Illustration: Fig. 143.--_Aspidura loricata_, a Triassic Ophiuroid. Muschelkalk, Germany.] The remains of _Articulate Animals_ are not very abundant in the Trias, if we except the bivalved cases of the little Water-fleas (_Ostracoda_), which are occasionally very plentiful. There are also many species of the horny, concentrically-striated valves of the _Estherioe_ (see fig. 122, b), which might easily be taken for small Bivalve Molluscs. The "Long-tailed" Decapods of the type of the Lobster, are not without examples but they become much more numerous in the succeeding Jurassic period. Remains of insects have also been discovered. Amongst the _Mollusca_ we have to note the disappearance, amongst the lower groups, of many characteristic Palæozoic types. Amongst the _Polyzoans_, the characteristic "Lace-corals," _Fenestella, Retepora_,[22] _Synocladia, Polypora_, &c., have become apparently extinct. The same is true of many of the ancient types of _Brachiopods_, and conspicuously so of the great family of the _Productidoe_, which played such an important part in the seas of the Carboniferous and Permian periods.