sixth tergites (see fig. 50). Ruedemann (1919, p. 92) has previously shown the alimentary canal in Carcinosoma newlini (Claypole), and Heubusch (1962, p. 222) has shown it in specimens of Eurypterus remipes lacustris Harlan.
Fig. 48. Central part of operculum of specimen PE 5094, Adelophthalmus mazonensis (Meek and Worthen). The spatulate lobes of the male are as well developed as those in the female.
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Specimens examined.—The specimens listed as H-1 to H-18 are in the private collection of Jerry Herdina. The specimen listed as "Witmer" is in the private collection of Harry Witmer, of Downers Grove, Illinois; the Vigo County, Indiana, specimen is in the collection of Michael Moore, of Tulsa, Oklahoma; and those listed as PE 3969, PE 5094, and PE 6263 are in the collection of Chicago Natural History Museum. All except the Indiana specimen were collected from the strip mines on the Will-Grundy County line, Illinois. All of the specimens collected by Herdina and the Bell specimen (PE 6263) are from the vicinity of the Santa Fe and the Gulf Mobile and Ohio railway tracks, where they cross the strip mines (see Richardson, 1956, pp. 6, 7). The specimen with the alimentary canal preserved is in the private collection of Francis Tully, of Lockport, Illinois; the copper cast, PE 6174, is in the collection of Chicago Natural History Museum.
Remarks.—The mode of preservation of eurypterids, particularly with reference to biometric studies, is of primary importance. It should be emphasized that the more incompetent the encasing material, the greater the amount of distortion that is found. To illustrate this point, the measurements of two carapaces of Slimonia acuminata (Salter) from the Silurian, Ludlow shales of Lesmahago, Scotland, are given (specimens in Princeton University):
| Specimen number | Length mm. |
Width at base mm. |
Width at midsection mm. |
| 1 | 103.5 | 96.0 | 88.0 |
| 2 | 150.5 | 80.0 | 77.5 |
The length/width-at-base ratio of No. 1 is 10.5:10 whereas that of No. 2 is 18.8:10. The latter carapace has been stretched but it
