In this way was established a perfect congruity of language. It was geographical in principle, and it represented the actual development of all our older rocks, which gave to it its true value and meaning.” The period, then, for the purposes of scientific description, may be divided into three sub-periods--the Upper and Lower Silurian, and the Cambrian. [36] _Quart. Jour. Geol. Soc_., vol. iii., p. 159. [Illustration: VIII.--Ideal Landscape of the Silurian Period.] [Illustration: Fig. 18.--Back of Asaphus caudatus (Dudley, Mus. Stokes), with the eyes well preserved. (Buckland.)] [Illustration: Fig. 19.--_a_, Side view of the left eye of the above, magnified, (Buckland.) _b_, Magnified view of a portion of the eye of Calymene macrophthalmus. (Hœninghaus.)] The characteristics of the Silurian period, of which we give an ideal view opposite (PLATE VIII.), are supposed to have been shallow seas of great extent, with barren submarine reefs and isolated rocks rising here and there out of the water, covered with Algæ, and frequented by various Mollusca and articulated animals. The earliest traces of vegetation belong to the _Thallogens_, flowerless plants of the class Algæ (Fig. 28), without leaves or stems, which are found among the Lower Silurian rocks. To these succeed other plants, according to Dr. Hooker, belonging to the Lycopodiaceæ (Fig. 28), the seeds of which are found sparingly in the Upper Ludlow beds. Among animals, the _Orthoceratites_ led a predacious life in the Silurian seas. Their organisation indicates that they preyed upon other animals, pursuing them into the deepest abysses, and strangling them in the embrace of their long arms. The _Trilobites_, a remarkable group of Crustacea, possessing simple and reticulated compound eyes, also highly characterise this period (Figs. 17 to 20); presenting at one period or other of their existence 1,677 species, 224 of which are met with in Great Britain and Ireland, as we are taught by the “Thesaurus Siluricus.”[37] Add to this a sun, struggling to penetrate the dense atmosphere of the primitive world, and yielding a dim and imperfect light to the first created beings as they left the hand of the Creator, organisms often rudimentary, but at other times sufficiently advanced to indicate a progress towards more perfect creations. Such is the picture which the artist has attempted to portray. [37] “The Flora and Fauna of the Silurian Period,” by John T. Bigsby, M.A., F.G.S. 4to, 1868. The elaborate and highly valuable “Thesaurus Siluricus” contains the names of 8,997 species of fossil remains, but it probably does not tell us of one-tenth part of the Silurian life still lying buried in rocks of that age in various parts of the world. A rich field is here offered to the geological explorer.[38] [38] Ibid, p. vi. LOWER SILURIAN.