The eyes of the Trilobites.

If, as is probable, to judge by the conformity of their cornea with that of recent crustacea, the trilobites like these were provided with crystalline cones beneath the corneal lenses or facets, only the latter have been preserved in a fossil state. Although the crystalline cones in consequence of their solid consistence might have been petrified as well as the cornea, they must, imbedded as these tiny cones lie, entirely wrapped up in delicate tissues, fall away and be lost, when the dissolution of the dead body had set in. Consequently the curious appendages on the inferior side of the lenses in Dalmanites vulgaris (Pl. III f. 50) or Phacops quadrilineata (Pl. V fig. 38) can noways be considered as belonging to the original structure of the eye, apart from their great dissimilarity with anything appertaining to the eyes of the Arthropoda. The cornea on the contrary cohered with the integument of the body, and it has been well preserved in a great number of trilobites.

I subdivide the trilobites in respect to the form of their eyes in the following manner:

I. Genera with compound eyes.

1. With prismatic plano-convex cornea facets.

Acidaspis, as a transitional form to the next group. Asaphus. Bumastus. Cyphaspis. Dysplanus. Encrinurus. Illænus. Megalaspis. Nileus. Phillipsia. Niobe. Proetus. Ptychopyge. Symphysurus.

2. With round or biconvex transversally elongate lenses.

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.

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.