[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 Euprooeps), 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