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Meek & Worthen. Am. Jour. Sci. & Arts. 1868. 46: 21, figure

Meek & Worthen. Geol. Sur. Illinois. 1868. 3: 544, figure

Hall. Pennsylvania 2d Geol. Sur. Rep't of Progress, PPP. 1884. p. 25, fig. 2, 3

Meek and Worthen have elaborately described and illustrated by an outline sketch this species from the iron stone concretions of the Carbonic of Mazon creek, Illinois. Their material consisted of but one specimen which exhibits the ventral side. Professor Whitfield kindly pointed out to us that the counterpart of this type is in the American Museum of Natural History and this has been made accessible for our study. The latter is a well preserved intaglio which happily complements the type. We therefore embrace this opportunity to add such data to the knowledge of this curious species as are furnished by this specimen. The most remarkable of these is the strong contraction of the postabdomen.

The excellent photograph reproduced on plate 26, figure 1 shows the condition of the specimen and all essential features observable in it. Some structures, as the position of the eyes, the outline of the impression of the opercular appendage, are only observable with favorable illumination. It has been possible to demonstrate the outlines of the carapace and first tergites a little better than shown in the photograph. These features as well as the outlines of the doublures of the tergites are illustrated by a text figure. For the sake of completeness we have also copied the outline drawing of the ventral side, given by Meek and Worthen.

Description. Body long, elliptical in outline, with the posterior third truncated by the abrupt contraction of the postabdomen. It is widest in the region of the fourth tergite, increasing gradually from the