Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/27

 still be of the same genus. On the other hand, Van Oyen (1956, pp. 60, 61) recognizes species and subspecies which easily can be demonstrated to be growth stages ("E. stylus Hall") or caused by poor preservation ("E. derbiensis Woodward"). Other American species, such as Adelophthalmus(?) potens (Hall), he also lumps together under the Bohemian A. imhofi, although the former probably does not represent the same genus or even the same family as the Bohemian form. Considerable evidence indicates that A.? potens (Hall) should be referred to the Hibbertopteridae.

Of the "Veine D" eurypterid carapaces figured and measured by Van Oyen (1956) the following are considered sufficiently well-preserved and complete to use for biometric studies:

¹ Not measured although complete and mainly undistorted.

² Erroneously measured by Van Oyen (1956): according to his photograph (fig. 131) and drawing (fig. 125), the correct measurements are 28.5 mm. in width at base and 21.5 mm. in length.

³ Obviously compressed and widened.

It is not the purpose of this paper to evaluate the conclusions reached by Van Oyen except where they concern the interpretation of the eurypterid in question or concern the genus. It has been recognized that many of the species described are much alike, at