Specimens of the Devonian trilobite Phacops rana milleri Stewart 1927 from the Silica Shale of Ohio are sometimes preserved in the early stages of the post-ecdysial cycle. Individuals that died in this early stage have pale, thin and wrinkled cuticles; in later stages the cuticle became rigid as it thickened and darkened. The post-ecdysial development of the cuticle and the schizochroal compound eye has been studied in a suite of specimens representing various stages, culminating in the intermoult condition. Scanning electron microscope studies on ethced specimens, supplementing cathodoluminescence and light microscopy, have enabled primary structures to be distinguished from secondary diagenetic effects, and have been used to elucidate diagenetic changes. In our earliest post-ecdysial cuticle the 10<latex>$\mu$</latex>m thick. The later thickening of the cuticle affects the principal layer alone. In subsequent development there is a division into three zones with a variety of vertical canals. The final intermoult cuticle can be up to 500 <latex>$\mu$</latex>m thick. In the developing eye, each of the post- ecdysial lenses has initially the form of a small, simple calcite cone hanging from the lower surface of the cornea. In later stages this lens spreads to the full width of the lens capsule, losing its canoical form, taking on a Huygensian shape, and eventually acquiring its mature form in which there is a central core of massive texture and, proximally, a thin intralensar bowl, of much the same appearnace as the core. The upper unit of the lens consists of thin calcite lamellae, radially arranged around the c-axis, each lamella consisting of palisade-like fibres, parallel in the lower part of the lens, but radiating outwards where they meet the convex outer lens surface. When the lens is mature, a ring of scleral materials forms on the inside of the cylindrical wall of the lens capsule, together with an annular girdle of fine-grained material at the junction of lens and sclera. Some suggestions are made regarding function in the developing eye, and the need for complete replacement of calcium carbonate at each ecdysis is discussed.