One of the most striking differences between the Lower Silurian and Cambrian periods is the great advance made by the Molluscs, in variety and numbers, during the former period. Molluscs exhibit the most significant change in the great expansion of the _Cephalopods_, a few of which had appeared in the uppermost Cambrian rocks. In the Lower Silurian period the Cephalopods became one of the predominant types in the marine life. In modern times nearly all Cephalopods are naked (_Cuttlefish_ and _Squid_), only a few having a shell (_Nautilus_ and _Argonaut_). The naked forms are higher creatures than those with shells. In the Lower as well as in the Upper Silurian periods no naked forms existed, but only the lower forms with shells (Nautiloids). In modern times and throughout later geological periods only Nautiloids with coiled shells were in existence. In both of the Silurian periods the shells were all straight, and the animals were called Orthoceratites. Such animals were extremely abundant in those times, and often reached an enormous size. Specimens have been found which were ten inches in diameter and over fifteen feet long. They were the most formidable animals of those early ages, and were the rulers and scavengers of the seas; so that the Lower and Upper Silurian periods are known as the =Age of Molluscs=. =Upper Silurian Period.= In this period great progress is made in the history of life on the globe. Upper Silurian life is the continuation and advance of the organic system that flourished in the Ordovician, certain groups expanding, others diminishing; and some new groups now appear for the first time. Sponges are still common. Among Cœlenterates the Graptolites have greatly diminished; Hydroid Corals have become important features of the seas and in the formation of the reefs. True Corals increase largely and play a more important part than in the preceding period. Honeycomb and Chain Corals are quite characteristic of this period. Among Echinoderms there is a diminution of the Cystids, and a marked increase of the Crinoids. A new class of Echinoderms, the Blastids, now make their first appearance. The Sea-urchins and Star-fishes have increased in variety and abundance. Brachiopods still exist in multitudes, but under changed forms. Among Molluscs the Orthoceratites are still the rulers of the seas. Among Arthropods the Trilobites are still numerous, though decidedly less so than in the preceding period. The land animals are insects of low types, mainly allied to cockroaches. Scorpions are also present as fossils in the Upper Silurian rocks. These animals prove the existence of a contemporaneous land vegetation.