[Illustration:  Fig. 64.—­Upper Silurian Trilobites. a, Cheirurus bimucronatus, Wenlock and Caradoc; b, Phacops longicaudatus, Wenlock, Britain, and America; c, Phacops Downingioe, Wenlock and Ludlow; d, Harpes ungula, Upper Silurian, Bohemia. (After Salter and Barrande.)]

Amongst the Articulates, we still meet only with the remains of Crustaceans.  Besides the little bivalved Ostracoda—­which here are occasionally found of the size of beans—­and various Phyllopods of different kinds, we have an abundance of Trilobites.  These last-mentioned ancient types, however, are now beginning to show signs of decadence; and though still individually numerous, there is a great diminution in the number of generic types.  Many of the old genera, which flourished so abundantly in Lower Silurian seas, have now died out; and the group is represented chiefly by species of Cheirurus, Encrinurus, Harpes, Proetus, Lichas, Acidaspis, Illoenus, Calymene, Homalonotus, and Phacops—­the last of these, one of the highest and most beautiful of the groups of Trilobites, attaining here its maximum of development.  In the annexed illustration (fig. 64) some of the characteristic Upper Silurian Trilobites are represented—­all, however, belonging to genera which have their commencement in the Lower Silurian period.  In addition to the above, the Ludlow rocks of Britain and the Lower Helderberg beds of North America have yielded the remains of certain singular Crustaceans belonging to the extinct order of the Eurypterida.  Some of these wonderful forms are not remarkable for their size; but others, such as Pterygotus Anglicus (fig. 65), attain a length of six feet or more, and may fairly be considered as the giants of their class.  The Eurypterids are most nearly allied to the existing King-crabs (Limuli), and have the anterior end of the body covered with a great head-shield, carrying two pairs of eyes, the one simple and the other compound.  The feelers are converted into pincers, whilst the last pair of limbs have their bases covered with spiny teeth so as to act as jaws, and are flattened and widened out towards their extremities so as to officiate as swimming-paddles.  The hinder extremity of the body is composed of thirteen rings, which have no legs attached to them; and the last segment of the tail is either a flattened plate or a narrow, sword-shaped spine.  Fragments of the skeleton are easily recognised by the peculiar scale-like markings with which the surface is adorned, and which look not at all unlike the scales of a fish.  The most famous locality for these great Crustaceans is Lesmahagow, in Lanarkshire, where many different species have been found.  The true King-crabs (Limuli) of existing seas also appear to have been represented by at least one form (Neolimulus) in the Upper Silurian.

[Illustration:  Fig. 65.—­Pterygotus Anglicus, viewed from the under side, reduced in size, and restored. c c, The feelers (antennae), terminating in nipping-claws; o o, Eyes; m m, Three pairs of jointed limbs, with pointed extremities; n n, Swimming-paddles, the bases of which are spiny and act as jaws.  Upper Silurian, Lanarkshire. (After Henry Woodward.)]