cision, and to demonstrate the entire homology in structure of the eyes of Pterygotus and Limulus. The visual surfaces of the specimens in question exhibit five distinct states of preservation, here illustrated by diagrams.
Figure 7 Compound eye of Pterygotus; diagrams of preservation states
In states I and II the visual surface is perfectly smooth without a trace of reticulation or lenticular depressions or prominences even under condensed light and under water. Two specimens show this state in the exterior view, two more as intaglios of the exterior of the head. Among the former is one of the best preserved carapaces [pl. 69, fig. 7] which distinctly shows the fine granulation of the surface and hence might be expected to retain also the finest details of the visual surface. The other specimens with smooth eyes are so well preserved that the corneal facets should be visible. Where the smooth surface is seen in an exterior view, the presence of the carbonaceous film indicates that the exterior of the test is actually under observation while in case II where the smooth visual surface is seen in a cast, the absence of the test proves likewise that we do not have before us an interior view of that surface.
The Second group comprises preservation states III and IV. In Case III the visual surface in an exterior view exhibits lenticular depressions in a network of squarish meshes. This is well shown in the large head [pl. 73, fig. 1]. In case IV the visual surface is seen in a cast and provided with a system of low papillae corresponding to the depressions seen in case III. Here the test is reduced to a mere carbonaceous film.
The relation between these two states of preservation, the absolutely smooth and the papillate, is indicated in the condition V exhibited by the fine head shield in the Buffalo collection. Here is a smooth visual
