Next in order of evolution we have the important tribe of the Paradoxidæ. These are preeminently distinguished from the Olenellidæ through the well developed facial suture, which without exception in them all runs outside the facial ridge and separates this from the free cheek.[12] This is a great step forwards in the evolution and establishes the fact, demonstrable also by other evidence, that the formation of the facial suture is subsequent to the appearance of the facial ridge. This preexisting ridge seems to have had no small influence on the development of the suture, it checked its progress from the front or from the sides toward the fixed cheeks and directed its course against the genal angles. It lay as a protection for the glabella against this disseverance, causing the separating line to run along its outside. [Footnote 12: They have thus a quinquepartite cephalic shield, as the later trilobites.] This group consists of the genus Paradoxides proper, as well as of Centropleura, Metadoxides and Hydrocephalus, if this is an adult form and not the larva of an unknown Paradoxides. Perhaps such forms as »Conocephalites» Emmrichi BARRANDE, as well as Anomocare limbatum, An. excavatum, Bathyuriscus and Dolichometopus may on account of the shape of their facial ridges be considered as related to the Paradoxidæ. But this must be left for coming researches to decide. Some American Cambrian forms also share in this characteristic and may upon closer inspection be ranged here. So Zacanthoides. In these as in the true Paradoxidæ the facial suture follows the ridge along its whole length, while in the trilobites of the third group the facial suture is in contact only with the posterior end of the ridge, the so called eye lobe. Remopleurides does not show characters, that as BEECHER thinks, could unite it with the Paradoxidæ. These are blind and Remopleurides has well developed eyes and an organization that gives it an isolated position in the system.