Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/19

 . 46. Schematic drawing of the ventral shield of Adelophthalmus mazonensis (Meek and Worthen) based on specimens H-2, PE 6263 and PE 3347.

margin and 3.8 mm. from the lateral margins. They are 2.8 mm. in length, 1.8 mm. in width, 6.7 mm. apart at the front, and 8.6 mm. apart at the back. The ocellar mound is located on the carapace 8.2 mm. from the anterior margin, 6.8 mm. from the posterior margin, and 9.3 mm. from the lateral margin; diameter, 0.8 mm.

Another well-preserved specimen (H-3) has the eyes located on the carapace, 4.2 mm. from the anterior margin, 7.3 mm. from the posterior margin, and 3.7 mm. from the lateral margins. The lateral eyes are 3.7 mm. in length, 2.0 mm. in width, 5.6 mm. apart at the front, and 7.3 mm. apart at the back. The ocellar mound is located on the carapace, 8.5 mm. from the anterior margin, 5.5 mm. from the posterior margin, and 9.0 mm. from the lateral margins. In an uncrushed condition, the ocellar mound is nearly round and measures approximately 1.3 mm. in diameter.

The largest carapace (H-6) has eyes that are 6.8 mm. in length and 4.5 mm. in width.

Although the details of the Type B operculum are known (Kjellesvig-Waering, 1948, p. 23, pl. 1, fig. 6), those of Type A are mainly known from an inconclusive outline given by Meek and Worthen (1868a, pp. 19–22), as the holotype is a dorsal impression and the shape of the operculum could only be surmised by its reflection through the mesosoma. Little can still be added to Meek and Worthen's original interpretation except to reveal that the