TRILOBITA (Neo-Lat. nom. pl., from Gk. Tpchc, trcis, three + X6,8os, lobos, lobe). An important group of extinct marine crab-like ani mals, whose fossil body shields are found in rocks of Paleozoic age in most parts of the world. The trilobites constitute a subclass of Crustacea, of which class they are very primitive members. The trilobite body was covered by a hard, calcareous dorsal shield and a thin ventral integument. The dorsal shield or carapace is of elliptical or oval outline and is divided by two dorsal furrows into three longitudinal lobes, the median axis and two lateral pleural lobes. This trilobite division, which characterizes this class of animals, gave the group its name. The shield is also divided into three transverse parts: the anterior head-shield or cephalon, the median thorax consisting of several movable segments, and the posterior abdominal shield or pygidium. The cephalon is generally of semicircular outline, and the posterior lateral corners, known as the genal angles, are often produced into spines. The more elevated median portion of the cephalon is the glabella, and this is separated from the cheeks or lateral portions by the usually well marked dorsal furrows. The surface of the glabella is broken up into a series of lobes by three pairs of transverse furrows, called the lat oral furrows. The paired lobes are called the lateral lobes and the unpaired anterior lobe, which is often very prominently developed, is known as the frontal lobe of the glabella. The cheeks are traversed in most trilobites by a sinuous line, the facial suture, along which the lateral portions or free cheeks break away from the fixed cheeks.
The eyes, which in most trilobites are of reni fo•m shape, are situated upon the cheeks. In some species (as Acidaspis mira ) the eyes are placed at the summits of high immovable peduncles; cer tain genera, as Agnostus, Microdiseus, and Ampyx, are totally blind; and other genera (as Trinucleus) have eye spots in their young stages and are totally blind when adult. Two
types of eyes are present—compound, situated on the free cheeks, and simple eye spots. oeelli or stemmata. The compound eyes are made up of lenses, which may vary in number from 14 in Trimerocephalus to 15,000 in 1-lemopleurides. The facial suture varies largely in its course across the eephalon, not only in the different species and genera, but also in the individuals that represent different stages of development of a single spe cies. Its position in the adult furnishes an ex cellent diagnostic character. In most trilobites a groove, called the occipital furrow, runs near to and parallel with the posterior margin of the cephalon, and cuts off the occipital lobe from the glabella.
The thorax is made up of several transverse segments which articulate with one another in such manner that some trilobites were able to roll themselves up into a ball after the manlier of the armadillo. The number of segments of the thorax is usually constant in the adults of each genus, but it varies within the subclass from two in Agnostus to 29 in 1-larpes. The pygidium, or caudal shield, is a solid plate formed by the union of several segments, of which traces are still evident in the ribs of its convex dorsal sur face. The axis and the pleural ribs are usually present, but in some species of Asaphus and 111amus they are almost entirely suppressed. The trilobite carapace is generally ornamented by small tubercles. Many genera have smooth or punctate tests; others have the test marked by concentric wrinkles; and in other genera spines are developed.