Like all arthopods, trilobites had compound eyes. The compound eyes in trilobites are among the first visual system evolved. Compound eyes in trilobites consists of a set of separated optical elements, the ommatidia. The internal structure of the ommatidia is largely unknown. There are two types of eyes found in trilobites: holochroal and chizochroal eye.
The holochroal eyes are characterized by a closely packed lens units. The whole visual surface is covered by one pellucide membrane or cornea. The lens are shaped like a truncated cone (Levi-Setti, 1993).
The chizochroal eyes are separately encased and each lans is maintained its position by cylindrical mounting, the sclara. Each lens is covered by its own cornea (Levi-Setti, 1993).
* both pictures are from (Levi-Setti, 1993)