Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/26

 the same dimensions as less compressed carapaces of A. mazonensis. This is in keeping with the overall narrow aspect of the entire opisthosoma of A. mansfieldi, in contrast to the robust construction of A. mazonensis. On the basis of the measurements of the carapaces above, my conclusions differ entirely from those advanced by Van Oyen (1956) in regard to these two species. Other morphological differences as stated on page 92 leave no doubt as to the validity of both species.

An examination of the ratios of both length/width-at-base and length/width-behind-eyes reveals that A. mansfieldi progressively becomes more narrow in the carapace with age as compared to A. mazonensis, whose ratios remain constant.

Van Oyen (1956, p. 38) states that his conclusions regarding the measurements of the eurypterids from the "Veine D" are based on the measurements of 130 carapaces. Of the length and width-at-base measurements of the carapace it is important to note that fully 107 (almost 86 per cent) of these carapaces should not have been measured, as they represented specimens that were either incomplete or obviously too distorted to permit a reasonable interpretation of the individual variation of the species represented by the prolific "Veine D" eurypterids. Van Oyen, therefore, presents an interpretation of the possible limits of distortion of 130 carapaces of the "Veine D" eurypterids rather than a criterion for the identification of the species based on actual individual variation. As a result, his so-called limits of variation lump together (1956, p. 59) nearly all of the North American species of the genus, giving a stratigraphic range from the Middle Pennsylvanian to the Middle Permian for a single species which he identifies as the Bohemian A. imhofi (Reuss). Nowhere is a highly specialized form such as an eurypterid known to encompass such a long range. Van Oyen completely disregards obvious morphological differences, as, for example, the under side of well-known forms such as A. mazonensis and A. mansfieldi, and both are included under A. imhofi. In order to reveal the fallacy of the conclusions advocated by Van Oyen, it might be permissible to compare the holotype of A. imhofi, which has a telson barely as long as the carapace, with an individual of A. mansfieldi of approximately the same size; the latter has a telson at least twice as long as the carapace. The latter also is obviously a much more highly spinous eurypterid, in contrast to the non-spinous character of A. imhofi. Nevertheless, Van Oyen groups both as the same species. Morphologically, I know of no two eurypterids that can be so different and