Page:Pennsylvanian invertebrates of the Mazon Creek Area, Illinois Eurypterida.djvu/15



This rare eurypterid was previously known from the holotype and seven specimens (Kjellesvig-Waering, 1948, p. 17). To this number can be added data from twenty-three specimens. Eighteen of these specimens were collected during ten years of intensive search by Jerry Herdina, of Berwyn, Illinois, and form part of his extensive collection. It includes the largest carapace known (see fig. 44).

Twenty-two of the specimens discussed here are from the spoil heaps of the abandoned strip mines of the Peabody Coal Company in Will and Grundy Counties, Illinois (Richardson, 1956). Several beds of coal have been exploited in these mines, but it is probable that the concretions bearing the famous Mazon Creek fauna (including these eurypterids) are derived from the Francis Creek shale member of the Middle Pennsylvanian (Westphalian C) Carbondale formation overlying Coal 2. The remaining specimen, from the Chiefton strip mine a few miles south of Terre Haute, in Vigo County, Indiana, is slightly younger. Spoil heaps of this mine are at present yielding large numbers of ironstone concretions to the same industrious collectors who have recovered so many fine specimens from the Illinois locality. According to Dr. Charles E. Wier, Head of the Coal Section of the Indiana Geological Survey, the concretion-bearing bed in the Chiefton mine lies above Coal VII, and is thus in the Shelburne formation (also Westphalian C).

The measurements of the holotype given by Clarke and Ruedemann (1912, p. 226) are erroneous as to the width of the carapace, which is given as 53.0 mm. This appears to be a typographical error, as the correct dimension is 43.0 mm, across the base of the carapace.

The new morphological data include the structure of the important ventral shield of the carapace, definite outlines of the spatulate plates of the Type A operculum, and the presence of the alimentary canal. Measurements of all carapaces are recorded for biometric comparisons. In this connection it should be noted that the condition of preservation of the carapace is of great importance