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.--_Prestwichia rotundata_, a Limuloid Crustacean. Coal-measures, Britain. (After Henry Woodward.)] [Illustration: Fig. 122.--Crustaceans of the Carboniferous Rocks. a, _Phillipsia seminifera_, of the natural size--Mountain Limestone, Europe; b, One valve of the shell of _Estheria tenella_, of the natural size and enlarged--Coal-measures, Europe; c, Bivalved shell of _Entomoconchus Scouleri_, of the natural size--Mountain Limestone, Europe; d, _Dithyrocaris Scouleri_, reduced in size--Mountain Limestone, Ireland; e, _Paloeocaris typus_, slightly enlarged--Coal-measures, North America; f, _Anthrapaloemon gracilis_, of the natural size--Coal-measures, North America. (After De Koninck, M'Coy, Rupert Jones, and Meek and Worthen.)] The _Crustaceans_ of the Carboniferous rocks are numerous, and belong partly to structural types with which we are already familiar, and partly to higher groups which come into existence here for the first time. The gigantic _Eurypterids_ of the Upper Silurian and Devonian are but feebly represented, and make their final exit here from the scene of life. Their place, however, is taken by peculiar forms belonging to the allied group of the _Xiphosura_, represented at the present day by the King-crabs or "Horse-shoe Crabs" (_Limulus_). Characteristic forms of this group appear in the Coal-measures both of Europe and America; and though constituting three distinct genera (_Prestwichia, Belinurus_, and _Euproöps_), they are all nearly related to one another. The best known of them, perhaps, is the _Prestwichia rotundala_ of Coalbrookdale, here figured (fig. 121). The ancient and formerly powerful order of the _Trilobites_ also undergoes its final extinction here, not surviving the deposition of the Carboniferous Limestone series in Europe, but extending its range in America into the Coal-measures. All the known Carboniferous forms are small in size and degraded in point of structure, and they are referable to but three genera (_Phillipsia, Griffithides_, and _Brachymetopus_), belonging to a single family. The _Phillipsia seminifera_ here figured (fig. 122, a) is a characteristic species in the Old World. The Water-fleas (_Ostracoaa_) are extremely abundant in the Carboniferous rocks, whole strata being often made up of little else than the little bivalved shells of these Crustaceans.