pressions of the opercular appendage; especially distinct is that of the anterior hastate part.
The ornamentation of the dorsal side is very striking. The carapace is for the most part smooth, but on the middle posterior region there appear small prominent tubercles most of which are semilunar and project backward. They are irregularly crowded around a median smooth depression. Along the posterior margin the semilunar scales are abruptly changed into two rows of larger, sharply angular scales. The first tergite is smooth; the following tergites as well as the postabdominal segments show a dense mass of extremely small semilunar to linear scales on the anterior half (mostly of so small size that these parts appear smooth), while the middle posterior halves bear prominent angular scales that are largest upon the median nodes. Toward the sides the angles of the scales become smaller until linear scars are formed, making a system of parallel oblique lines on the epimera (not well shown in the figures).
Measurements. The carapace is 32 mm long and approximately 53 mm wide at the base. The eyes were 5.5 mm long and 16 mm apart. The preabdomen measures 46 × 62.5+ mm. The first tergite is 4 mm long; the second 7.5 mm long and 59 mm wide; the third is 9.5 mm long and 61+ mm wide. The first postabdominal segment measures 9 × 52 mm, the next 11.5 × 26+ mm. The limbs are only fragmentary.
Horizon and locality. Lower Coal Measures of Mazon Creek, Grundy co., Illinois. Counterpart in American Museum of Natural History (no. 8532).
Genus EUSARCUS Grote & Pitt
The most striking and interesting eurypterid of the waterlimes of Buffalo is undoubtedly a large animal which, when discovered in 1875, was made the type of a new genus and species by Grote and Pitt under the designation of Eusarcus scorpionis. The authors of the genus gave no generic diagnosis but the following description of the species suggests what they considered as its distinctive characters:
The cephalothoracic portion appears to be separate from the body, and to be considerably narrower in proportion than in allied forms. The
