trilobite

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of numerous extinct marine arthropods of the class Trilobita, of the Paleozoic Era, having a segmented body divided by grooves into three vertical lobes and found as fossils throughout the world.

Century Dictionary and Cyclopedia

1. n. Any member of the Trilobita: so called from the three lobes or main divisions of the body—cephalic, thoracic, and abdominal. See Trilobita. Trilobites are of much popular as well as scientific interest; some of them occur in profusion in Paleozoic formations, and trilobites as a group are among the longest and most widely known of fossils, not yet entirely divested of a problematical character. In the Linnean system all of the few forms then known were considered one species, named Entomolithus paradoxus, and a sort of likeness to chitons caused Latreille to range these organisms near those mollusks. Trilobites are the most characteristic fossils of their class throughout the Paleozoic rocks. More than 500 species have been described, and upward of 70 genera have been named and referred to several higher groups. Upward of 300 species, of about 50 genera, mostly of the Cambrian and Silurian, are described as British; 350 species, of 42 genera, are recorded from the lower Paleozoic rocks of Bohemia; the Devonian forms are comparatively few; and the series closes with some small Carboniferous species, mostly of two genera. The oldest genus is named Agnostus. Some of the trilobites are of comparatively gigantic size, as species of Paradoxides, 2 feet long. An ordinary trilobite, a species of Dalmanites, is figured above. The body of a trilobite is generally of a flattened oval figure, whose upper side presents, besides the obvious transverse division into three parts, a median longitudinal elevation from one end to the other. The head, composed of several coalesced segments, and presenting certain sutures, constitutes a cephalic shield rounded in front, with an axial raised section, the glabellum, on each side of which are large compound eyes (not unlike those of the horseshoe-crab), and whose lateral limbs or borders are prolonged backward to a varying distance on each side of the thorax (in some cases produced beyond all the rest of the body). The second division of the body consists of a varying number (up to twenty-six) of separate thoracic segments, which were more or less freely movable upon one another, so that some trilobites could roll themselves up in a ball, like a sowbug (isopod) of the present day. The raised axis of the thoracic division is the tergum, and parts on each side of it are the pleura. The third division of the body is the abdomen or pygidium, of a variable number (up to twenty-eight) of segments, in general resembling the thoracic segments, and with an axial raised portion, but united together. Of the under surface of a trilobite almost nothing was known until recently, and much still remains to be accurately determined. A well-developed lip-plate or hypostome had been recognized, but nothing further was known until 1870, when the under side of a species of Asaphus, showing indistinct appendages, was described by Billings. Other investigators have pursued this subject, by means of sections of fossils, with the result of showing the presence of articulated appendages, or legs, and of other organs regarded as gills. The embryology of trilobites, so far as known, agrees most nearly with what has been accurately determined in the ease of the horseshoe-crab. What may be inferred of the mode of life of trilobites is that probably their habits were like those of these crabs.

Wiktionary

1. n. biology An extinct arthropod of the class Trilobita, whose body had three large lobes.

GNU Webster's 1913

1. n. (Paleon.) Any one of numerous species of extinct arthropods belonging to the order Trilobita. Trilobites were very common in the Silurian and Devonian periods, but became extinct at the close of the Paleozoic. So named from the three lobes usually seen on each segment.

WordNet 3.0

1. n. an extinct arthropod that was abundant in Paleozoic times; had an exoskeleton divided into three parts