never been observed, not even in the transparent specimens studied by Holm, and the visual area occupies only a kidney or crescent shaped space on the outer slope of the eye node.
In Hughmilleria we have found the following condition of the compound eyes. The visual area, in outer view, is always smooth in the hundreds of carapaces we have seen, and without any trace of facets, but an interior view shows unmistakable traces of the facets. These structures are hence present, though very feebly developed.
The compound eye is arcuate on the outer side, but angular on the inner side, as correctly described by Sarle. A laterally compressed specimen [pl. 60, fig. 7] has permitted us to obtain an unforeshortened view of the eye and this shows the kidney-shaped visual area and a triangular palpebral lobe on the inside which is the cause of the angularity.
In H. shawangunk the eyes appear marginal in many specimens, because the marginal portion of the carapace was so impendent that in specimens vertically compressed it becomes folded upon itself and only a portion of the eye is shown [pl. 65, fig. 9]. Wherever the whole carapace is flattened out, the lateral eye is intramarginal by a considerable interspace [pl. 65, figs. 3, 4, 12]. In Pterygotus the lateral eye, at least in the mature stage, is always truly marginal.
Thus we infer that the eyes of Hughmilleria exhibit transitional or intermediate features between Eurypterus and Pterygotus.
The body of Hughmilleria is slenderer than that of the typical Eurypterus, but it falls fully within the limits of the recognized variations of the genus Eurypterus and is much surpassed in slenderness by such forms as E. maria. Similarly the metastoma, while exhibiting a decided tendency to a cordate form, is not nearly so pronounced as in Pterygotus and in general form, stands, about midway between that of Eurypterus and Pterygotus.[1]
The opercular appendages have been considered by Sarle [op. cit. p. 1091] as corresponding in a general way to those of Eurypterus; we see in
- ↑ It also seems to be subject to considerable variation as evidenced by figure 4, plate 62, and figures 3, 4, plate 66.
