Trilobite (Gr.τρείς, three, and λοβός, lobe), the name of a group of fossil crustaceans, so called from the three lobes into which the body is divided. They do not correspond exactly to any living group, but, according to Burmeister ("Organization of Trilobites," Ray society's publications, 4to, London, 1846), were a peculiar family of crustaceans, nearly allied to the existing phyllopoda (like apus and branchipus), and forming a connecting link between these and the entomostracan poecilopoda (like ar-gulus, caligus, and other parasites called fish lice); they come nearest to phyllopods, especially in the double large eyes, undeveloped antenna, and soft membranous feet, and nearest of all to branchipus; a marked resemblance in the form of the limulus (king crab, or common horseshoe of our coasts), especially the larva, is also observed to that of many species of trilobites. (See King Crab.) The general form of the animal is oval, divided into three well defined regions, the head or buckler, the thorax, and the abdomen or py-gidium, the last two composed of semicircular plates or segments, varying in number, by whose movements the animal could roll itself into a ball like the common wood louse and pill bug (oniscus and armadillo). Each of these three divisions presents three lobes limited by two longitudinal depressions; the head is generally the largest and considerably the widest, varying from one fourth to one half the total length, semicircular, with a border often ornamented with granulations, depressions, and spines; the middle portion is the glabella, the grooves which mark its lateral limit corresponding, according to Barrande, to the insertion of the jaws or first pair of feet; the different pieces are united by distinct sutures, which are important zoological characters.

Eyes have been denied to some genera; some had eyes when young, but lost them when old; others had two well formed, compound, facetted, prominent eyes, -which are often perfectly preserved in the fossil state; they are sometimes larger than half the length of the head, the greatest diameter being almost always the longitudinal; they had no simple eyes. Traces of a mouth have been distinguished in a few'; no traces of antenna) have been found, and they were probably short and feebly developed. The number of the thoracic segments varies in different genera and at different stages of growth, but is constant in adults of the same species; the terminal portions on the sides are the pleuroe, and are curved backward and sometimes very long; traces of nine pairs of feet have been discovered, and they were doubtless soft, membranous, and leaf-shaped, as in phyllopoda. The pygidium was made up of segments like those of the thorax, but consolidated to form a posterior buckler; it was usually semicircular, less long than wide, developed inversely to the thorax, and largest in the more recent genera.

The shell had a thinner horny membrane covering it, becoming more delicate toward the median line; between the two is found in the fossils a stony layer measuring their distance from each other; the lower surface was soft and membranous; the skin was undoubtedly cast as in other articulates, and Wahlenberg has suggested that some supposed new species may have been founded on their cast shells. They have been divided into three families, according to the nature of their covering: 1, eurypteridoe, without shell, including the single genus eurypterus (De Kay); 2, cytherinidoe, with bivalve, bean-shaped shell, including the single genus cytherina (Lam.); and 3, trilo-hitce, with a shell having as many rings as there are joints to the body, containing many genera and species, and divided into two large groups, one with the power of rolling into a ball, like calymene, and the other with no such power, as in ogygia. According to Bur-meister, the trilobites moved only by swimming, just below the surface of the water, with the back downward, rolling into a ball when danger threatened from above, and did not creep upon the bottom; they lived in shallow water, near the coast, associating in immense numbers, chiefly of the same species; while only six or eight species occur in a given stratum, the number of individuals was very great; their food consisted of small aquatic animals and their spawn; they underwent progressive metamorphoses, and varied considerably according to age; their metamorphoses are given at length by Barrande, who makes four distinct types, according to the serial development of the different parts. - Trilobites are among the oldest of the articulata; though none are now living, during the palaeozoic period they were very abundant, and almost the only representatives of their class.

They have been most studied in Bohemia, and by M. Barrande. None are found above the carboniferous rocks, and only two or three in them. Barrande's primordial fauna, or the lower Silurian, has one genus but no species passing to his second fauna or middle Silurian, and this has many genera but no species common to it and the third fauna or upper Silurian, which in turn has several genera passing to the Devonian fauna - the whole series affording remarkable proofs of the limitations of faunas in time; the distribution of particular genera and species in space was also very circumscribed, probably on account of their feeble locomotive powers. In America several trilobites, especially paradoxides and its allied genera, have been met with in slates formerly classed among the metamorphic rocks, as the P. Harlani (Green), found in Braintree, Mass., in 1856, by Prof. W. B. Rogers, and this and other trilobites found in Canada and Newfoundland. - The trilobites have long attracted much interest, as well on account of the great numbers in which they have been found in many localities, as from their singular conformation, and the perfect state in which their forms are preserved.

The eye is very beautiful, and its perfection in many of the stony fossils, especially some brought from the Hartz mountains, and from the upper Silurian limestone of Dudley, England, is very remarkable; the facets or lenses, sometimes nearly 400 in number, are like those observed in the eye of the dragon fly and butterfly, and as in these insects are arranged around a conical tube through which the visual rays enter from almost every direction; in the asapJius caudatus each eye thus has a range of nearly three fourths of a circle, and both together command a panoramic view. The structure of the eye also indicates the prevalence in those ancient periods of the same conditions of the waters and the atmosphere, as regards their adaptation to the organs of vision, as now obtain. - The geographical range of trilobites is very extensive; these fossils are met with at most distant points, both of the southern and northern hemispheres; they are found all over northern Europe, and in numerous localities in North America, in the Andes of Bolivia, and at the Cape of Good Hope. Trenton Falls, N. Y., has afforded, in the limestone known by its name, fine specimens of the species ccdymene Blumenbachii (Brongn.). Lebanon, Ohio, is another interesting locality.

In Adams co., Ohio, Dr. Locke procured an iso-telus, to which he gave the specific name me-gistos, that measured more than 20 in. in length and 12 in. in width; the I. gigas and paradox-ides Harlani have been found more than 12 in. long. (See "American Journal of Science," 1871, p. 228, and 1872, p. 268).