ACIDASPIS, Asq-das'pls (Gk. akis, spine tiaric,aspis, shield). A peculiar genus of trilobites found in rocks of Silurian and Devonian age in nearly all parts of the world. The individuals are, as a rule, small, and are .remarkable because of the spiny ornamenta tion of the dorsal shield or carapace. The lobo tion of the head shield is rather peculiar and quite unlike that seen in any other genus of trilobites, the trilobite division being obscured by a number of supplementary furrows and by the strong development of two longitudinal false furrows between the normal dorsal furrows. The thorax eontains nine or ten segments, and the tail-shield is of rather small size. In some species a row of slender spines is developed upon the sides of the head-shield and a long spine projects from each posterior angle. Besides these there are often two long straight or curved spines direeted upward and backward from the middle posterior edge of the head. Each seg
ment of the thorax is produced laterally into long spines, and there are also two short spines on the raised median portion of each segment. The tail-shield is in nearly all species likewise furnished with spines, so that on the whole these animals must, though of small size, have presented a rather formidable aspect to larger animals which sought to prey upon them. A species of the genus are of particular interest on account of the :ilatottnal deNelopment of the eyes, which are placed at the summits of highly elevated slender, though immovable, stalks, which arrangement enabled the animal to vont mond a view in all direetions. This elevation of the eve reealls the stalk-eyes of some modern crabs and lobsters. For illustration, Plate of TRILOBITES.