The fossils of this system are very numerous, but consist mostly of the lower forms of animal life. _Corals_ (Figs. 8 and 9) are abundant, and constitute in some places a large proportion of the limestones. The _Crinoidea_, or lily-shaped animals, consist of a jointed stem permanently attached, and bearing at the free extremity of the stem an expanded portion, which is the pelvis, or digestive cavity. The mouth is surrounded with a series of leaf-like tentacula, which serve the purpose of seizing and holding food. Fig. 10 represents the pelvis of one of the silurian fossils. The general character of the animal is better represented by Fig. 30. The most abundant fossils of this period are the lowest orders of _bivalve mollusca_ (Fig. 11). The _Cephalopoda_ are characterized by having the organs of locomotion attached to the head. The shell of several species is peculiar in being divided into distinct cells, or chambers (Fig. 12, _b d_), perforated by a tube (siphuncle _a_). These fossil shells are sometimes straight, as the _Orthoceras_ (Fig. 13), or curved, as shown in the several forms of Fig. 14. The _Trilobite_ was an articulated, crustaceous animal, having two lines along the back dividing it into three lobes, from which circumstance its name is derived. It is found in great numbers in the Silurian rocks (Fig. 15). In a few instances remains of fishes have been found, but they by no means characterize the system. [Illustration: Fig. 11.] [Illustration: Fig. 12.] [Illustration: Fig. 13.] [Illustration: Fig. 14.] [Illustration: Fig. 15.] The geographical range of this system is probably greater than that of any system of rocks above it. It is found occupying a large part of the territory west of the Alleghany Mountains, from Canada, through New York, and the other states, to Alabama; and extending westward to and beyond the Mississippi river. It occupies a large district in the west of England, and is found in great force in the north and east of Europe. 2. _The Old Red Sandstone._--This formation consists almost entirely of a sandstone of a red color. It admits of division into three parts, though the characters vary in different places. The lowest is a thin-bedded argillaceous sandstone, consisting of finely levigated material, and easily splitting into thin sheets. From this circumstance it has received the name of _tilestone_. The middle portion is composed of nodules or concretions of limestone imbedded in a paste of red sand and shale. This has been called by English geologists, _cornstone_, and though very partially developed in some regions where the system is found, it is yet a very persistent member. The highest member of this formation is a mass of red sandstone, often passing into a coarse _conglomerate_. In England the thickness of the Old Red Sandstone is not less than ten thousand feet. In this country it is scarcely three thousand feet. [Illustration: Fig. 16.]