The Ringed Worms (_Annelides_) are abundantly represented in the Lower Silurian, but principally by tracks and burrows similar in essential respects to those which occur so commonly in the Cambrian formation, and calling for no special comment. Much more important are the _Articulate_ animals, represented as heretofore, wholly by the remains of the aquatic group of the _Crustaceans_. Amongst these are numerous little bivalved forms--such as species of _Primitia_ (fig. 47, f), _Beyrichia_ (fig. 47, e), and _Leperditia_ (fig. 47, i and j). Most of these are very small, varying from the size of a pin's head up to that of a hemp seed; but they are sometimes as large as a small bean (fig. 47, i), and they are commonly found in myriads together in the rock. As before said, they belong to the same great group as the living Water-fleas (_Ostracoda_). Besides these, we find the pod-shaped head-shields of the shrimp-like Phyllopods--such as _Caryocaris_ (fig. 47, d) and _Ceratiocaris_. More important, however, than any of these are the _Trilobites_, which may be considered as attaining their maximum development in the Lower Silurian. The huge _Paradoxides_ of the Cambrian have now disappeared, and with them almost all the principal and characteristic "primordial" genera, save _Olenus_ and _Agnostus_. In their place we have a great number of new forms--some of them, like the great _Asaphus tyrannus_ of the Upper Llandeilo (fig. 47, a), attaining a length of a foot or more, and thus hardly yielding in the matter of size to their ancient rivals. Almost every subdivision of the Lower Silurian series has its own special and characteristic species of Trilobites; and the study of these is therefore of great importance to the geologist. A few widely-dispersed and characteristic species have been here figured (fig. 47); and the following may be considered as the principal Lower Silurian genera--_Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus, Lichas, Illoenus, Æglina, Harpes, Remopleurides, Phacops, Acidaspis_, and _Homalonotus_, a few of them passing upwards under new forms into the Upper Silurian.