The metastoma is very similar to that of E. remipes, but subject to some variation and frequently somewhat contracted in the posterior portion [pl. 11, fig. 5], giving it a more slender appearance.
The female genital appendage was very correctly figured by Hall [loc. cit. pl. 81B, fig. 4]. Several well preserved specimens before 
Figure 35 Eurypterus lacustris Harlan. Cast of interior of proximal portion of female opercular appendage us show the great size attained by this organ in mature individuals. It reaches with the long paired, hornlike terminal appendages, to the posterior edge of the third sternite. The interior tubular appendages of this organ are finely seen in the original of plate 12, figure 2, where the integument has broken away. They have the precise form and relative proportions of that of E. remipes. The appendage of the second sternite has not been seen unobscured by the overlying appendage of the operculum. It is, however, well shown in the interior view, reproduced in plate 11, figure 4 
Figure 36 Eurypterus lacustris Harlan. Young individual. Reproduced on plate 11, figure 1. Natural size [Hall's type of pl. 81, fig. 6]. It is extremely slender and pointed and reaches nearly as far back as that of the operculum. The male appendage does not differ from that of the preceding species.
Ornamentation. As far as our observation goes, the ornamentation is quite like that of E. remipes. This is especially notable in regard to the series of scales on the dorsal side, already correctly figured by Hall [pl. 81B, fig. 1]. The smaller scales which are rarely seen, are crescent-shaped to subtnangular and nowhere densely crowded. Those of the longitudinal series were larger and circular in the anterior portion, becoming elongate in the posterior part of each row.
Ontogeny. But little is known of the ontogeny of this species as but very few young individuals have been obtained and these are too far developed to afford much information. Hall's figure, [plate 81, figure 1],
