Acerocare. Bronteus. Chirurus. Ctenopyge. Cyrtometopus. Eurycare. Peltura. Sphærophthalmus. =II. Genera with aggregate eyes of biconvex lenses.= Acaste. Chasmops. Dalmanites. Phacops. =III. Genera with isolated eyes, one or several stemmata at the extremity of a straight facial ridge.= Harpes. Harpides. (?) Trinucleus in the larval state. Only a few authors have before now occupied themselves with the intimate structure of the trilobite eye. PACKARD gave in 1880, in the »American Naturalist» a note on the structure of the eye of trilobites (p. 503). There are some rough and inexact sketches of the eyes of Limulus and Asaphus, and although he seems to have known the beautiful researches of GRENACHER he still »claims that the trilobite eye was organized on the same plan as Limulus». This statement is altogether wrong, and as I hope to show the trilobites have had eyes entirely different from that of Limulus and instead agreeing with those of the Isopoda and perhaps also with a few other Crustacea. In 1889 J. M. CLARKE published an account[28] on the »Structure and development of the visual area in the trilobite Phacops rana GREEN». The aggregate eye described by him are of the type forming my third group. His holochroal division embraces my first and second groups and the schizochroal my third. [Footnote 28: Quart. Journ. of Morphology, vol. II p. 253.] The latest contribution to the knowledge of these eyes is found in EXNER'S »Physiologie der facettirten Augen von Krebsen and Insecten», 1891, where he gives good figures of the lenses of Phacops fecundus, pl. II figs 18, 19. He says that the palpable difference in the structure of these eyes and those of Limulus point to a change in the function of these eyes. I. =Compound eyes.= =1. Eyes with prismatic, plano-convex lenses.= A pellucid, smooth and glossy integument, a direct continuation of the common test of the body covers the corneal lenses, quite as is the case in so many of the recent crustacea. In the plurality it is, however, difficult to discern the lenses from the outside.