332 . NEW YORK STATE MUSEUM 
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 struc- 
tures 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 speci- 
men [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 tri- 
angular palpebral lobe on the inside which is the cause of the angularity. 
In H. shawangunk the eyes appear marginal in many speci- 
mens, 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 Euryp- 
terus, 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 Ptery- 
gotus and in general form, stands, about midway between that of 
Eurypterus and Pterygotus." 
The opercular appendages have been considered by Sarle [op. cut. 
p. rog1]as corresponding in a general way to those of Eurypterus; we see in 
1It also seems to be subject to considerable variation as evidenced by figure 4, 
plate 62, and figures 3, 4, plate 66. 
