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corals, figs. 1 and 2, are the Cup-leaf (Cyathophyllum), and the Chainpore (Halysites) species. These occur in the oldest strata in a state of preservation as complete as if they had but yesterday been taken from the ocean. Figs. 3 and 4 represent silurian crustacea, jointed animals like the common crab, lobster, shrimp, woodlouse, &c. Fig. 3 is the wellknown “Dudley trilobite,” or “Dudley locust.” The name Calymene, or obscure, was given to it when much doubt existed among collectors as to its true nature. Fig. 4, Ogygia, takes its name from the same circumstance - Ogyges was a Grecian monarch who reigned at the earliest and obscure period of history. Representatives of the highest class of mollusca meet the geologist in his first researches among the early fossiliferous strata, figs. 5 and 7. These were animals whose organs of locomotion were attached to the head (Cephalopoda) like the Nautili of Southern seas. They inhabited a chambered shell. Figs. 6, 8, 9, represent other molluscan forms of the silurian period. Atrypa occurs from the Lower Silurian to the Trias; Pentamerus is characteristic of the Upper Silurian and the Devonian, and the whorled Euomphalus ranges from the Lower Silurian to the Trias.
Graptolites are characteristic silurian fossils. They are the remains of zoophytes which have left their impressions on rocks formed of fine
mud in the bottom of silurian seas. Bearing some resemblance to the sea-pens of the present epoch, the different species have been grouped under the name Graptolithina (grapho, I write, and lithos, stone). They have been classified as—1. Graptolites, G. priodon, fig. 7; 2. Diplograpsus, in which the cells are arranged on both sides of a central stem, D. folium, fig. 8; 3. Didymograpsus, or double-branched graptolites, D. Murchisonii, fig. 9; and 4. Rastrites, or rake-like graptolites, R. peregrinus, fig. 10. At the top of the silurian system lies a series of rocks known as the Lower Ludlow, Aymestry, and Upper Ludlow rocks, so named from places in Shropshire, near which
they are largely developed. In the Lower Ludlow one of the earliest forms of the star-fish (Asteria) appears—Protaster Miltoni, fig. 11– bearing some resemblance to the brittle star-fishes (Ophiuræ) of our seas.
Among the numerous molluscan remains of the silurian period the bivalves, Orthis calligramma, fig. 12, Modiolopsis expansa, fig. 13, and Avicula Danbyi, fig. 14, may be noticed.
The first two are met with in the Lower, the third in the Upper Silurian rocks. In the Lower Silurian rocks of Scotland and of Canada a
genus of belly-footed (Gasteropoda) shells occur, known as Maclurea. One of these, M. Logani, fig. 15, is characteristic of North American strata.
The highest class of mollusca (Cephalopoda) was well represented in the silurian period. Of these the British Lituites cornu-arietis, fig. 16, or Trumpet-formed ram's horn shell, may be named in addition to those represented on Plate I.
Two species of trilobites are also given on Plate I. One of them (Calymene) belongs to the Upper, the other (Oxygia) to the Lower Silurian. Another highly characteristic form, Phacops caudatus, fig. 17, or the Pea-eyed trilobite, is found in the different members of the Upper Silurian series. This form is noted because of the highly complicated structure of its compound eyes, which indicate that it lived in conditions of light similar to those enjoyed by the crustaceans of our seas.
Lying above the Upper Silurian is the Devonian or Old Red Sandstone, to the correct knowledge of which the late Professor Fleming was among the first to introduce geologists. It has, moreover, become forever associated with the name of the late Mr. Hugh Miller, who devoted the vigour of his life and his great attainments to its elucidation. It is generally divided into lower, middle, and upper old red sandstones. In the lower may be included the Devonian, properly so called from its great typical development in Devonshire. At the bottom of the true Scottish member of this series occurs what is known as “the great pebbly conglomerate.” This layer is now referred to for the sake of calling attention to the rate of deposition of certain great strata. It is formed mainly of coarse fragments of granitic rocks, and of the mud obtained by the wear of these, as they
Fig. 17. were driven against each other and tossed to and fro in a wild weltering ocean. “A vast stratum,” says Mr. Miller, “ of water-rolled pebbles, varying from a hundred feet to a hundred yards, remains in a thousand different localities, to testify of the disturbing agencies of this time of commotion. The hardest masses which the stratum incloses —porphyries of vitreous fracture that cut glass as readily as flint, and masses of quartz that strike fire as profusely as steel, are yet polished and ground down into bullet-like forms, not an angular fragment appearing in some parts of the mass for yards together. The debris of our harder rocks, rolled for centuries in the beds of our more impetuous rivers, or tossed for ages along our more exposed and precipitous sea-shores, could not present less equivocally the marks of violent and prolonged attrition than the pebbles of this bed.” When the time of calm came, the heavier portions would settle down first, the coarser mud would fill up the gaps between the rounded pebbles, and the finer and lighter earthy matter would subside last. Thus it is still found in similar circumstances. But this whole process is suggestive of modifications on the current method of calculating the time necessary for the deposition of rocks. The prevalent error on this point consists, not so much in claiming immense periods for the formation of the fossiliferous strata of the earth's crust, as in determin