AS'APHUS (Gk. a, a priv. aaddic, saphOs, clear, distinct). An Ordovician genus of smooth trilobites, often of large size, among the mem bers of which there is manifested a tendency to lose that longitudinal lobation which is so pe culiarly characteristic of the trilobites as a group. The large head-shield has the central portion or glabella indistinctly limited, and is provided with two reniform compound eyes which in some species are placed upon immovable stalks that, rising far above the general surface of the head, enable the animal to command a view of the entire horizon. This latter arrange ment is of interest when considered in connec tion with the nature of the joint that connected the head with the thorax, this joint having in all trilobites been so constructed as to prevent, any lateral motion of the head-shield, a slight ver tical motion only having been possible. The thorax has eight segments, and the tail-shield, of a size somewhat less than that of the head, is generally quite smooth and nonsegmented.
The allied genus Megalaspis has a head of elongated triangular form, with the anterior and lateral angles produced into sharp spines. An other allied genus, Niobe, which appeared at a somewhat earlier age than did Asaphus, has the longitudinal lohation quite well !narked. and seems to indicate the form of the stock from which Asaphus was evolved through a gradual loss of the t•ilohntion and segmentation of the carapace. These three genera, together with four others of less importance and over 150 species. comprise the family Asaphidae, which is found abundantly in all portions of the Ordo vician system in Europe and North America. Sec OttoovalAN; TRILOBITA ; and for illustration see plate aceompanying TRILOISITA.