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.