Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/17

 in biometric studies; therefore the state of preservation is given with each specimen. The dimensions of these carapaces are as follows:

The ventral shield (fig. 46) comprises a broad plate, not unlike that of Hughmilleria, but without trace of any sutures except a longitudinal one dividing the plate into two halves, longitudinally, along the anterior. This suture is present in Hughmilleria (Størmer, 1934, fig. 2). A very narrow, raised, marginal rim surrounds the carapace. A small, deep, triangular indentation occurs at the middle of the anterior of the shield to receive the triangular process at the anterior of the carapace, thereby forming a locking device for the ventral shield.

The ventral shield is ornamented with very fine semi-lunar scales that point away, or radiate, from the position of the mouth (see also fig. 45). It was almost entirely preserved in specimen PE 6263 and partly preserved in specimens H-2 in the Herdina collection and PE 3347.

Specimen PE 5094 (a male, Type A) has the eyes located on the carapace, 4.9 mm. from the anterior margin, 7.7 mm. from the