In the earliest fossiliferous rocks the most abundant and important Arthropods are the Trilobites (Fig. 81), an extinct group which appears to have been related to the primitive Crustacea. The name Trilobite refers to the three-lobed form of the body when seen from the dorsal side, most species having a pair of grooves running lengthwise which divide off a middle lobe containing the principal organs of the body from two lateral "pleural" expansions covering the limbs. The head-shield shows indications of being composed of five segments, and bears a pair of sessile compound eyes. It is followed by a number (up to twenty-six) of free somites, and the body ends in a tail-shield, or "pygidium," which is often plainly composed of several somites fused together. Although Trilobites are among the commonest and most familiar of fossils in the older rocks, the nature of their appendages remained quite unknown until within recent years, when specimens of several species showing the structure of the limbs and under-side of the body were discovered in America. From these it appears that the head bore in front a pair of long thread-like antennæ and four pairs of two-branched appendages, each with a jaw process, or "gnathobase," turned towards the mouth, which is covered below by a large anterior lip, or "hypostome." It seems probable that the five pairs of head-appendages correspond respectively to the antennules, antennæ, mandibles, maxillulæ, and maxillæ, of Crustacea; but the second pair appear to have acted as jaws, retaining the gnathobase which, among Crustacea, is only hinted at by the hooked spine on the antenna of the nauplius larva. Each of the free somites and of those forming the tail-shield bears a pair of two-branched appendages, not differing greatly from the posterior appendages of the head, but becoming smaller and more flattened towards the hinder end of the body. The numerous genera and species of Trilobites present great differences in the form and ornamentation of the dorsal surface of the body, and it is probable that considerable differences may also have existed in the structure of the limbs, which are only known in two or three species. Some Trilobites are among the most ancient of known fossils, being found in rocks of the Lower Cambrian epoch. The group reaches its maximum development in the Ordovician, and the number of the species and size of the individuals gradually diminish through the Silurian and Devonian till they become extinct at the close of the Carboniferous epoch, except for a single species found in rocks of Permian age in America.