ple extremity of the mesial appendage of its operculum, as well as in the possession of two little spatulate supplementary pieces (ss). Hence we very strongly suspect that other characters will be found, when better specimens can be studied, showing it to belong to a distinct sub-genus, if not indeed to an entirely distinct genus from Eurypterus proper, in which case we have proposed for it the name Anthraconectes."
Whether these differences noted are of sufficient importance to constitute generic or subgeneric distinction may perhaps admit of different opinion, depending upon the interpretation of the relative value of variations of certain parts of the organism.
A specimen from Mazon creek for many years (from 1860) in my own collection and now in the American Museum of Natural History in New York City, appears to be not only identical with this one, but a counterpart or impression of the specimen described by Messrs. Meek and Worthen (see Fig. 3). It is less complete in its appendages, but the form and proportions of the body and the measurement of the parts correspond in all particulars. The mesial appendage of the thoracic plate extends to the sixth segment, as shown by a median ridge extending to that point, but it cannot be determined whether the posterior extremity is simple or bifurcate. The lines indicating the anterior attachment of this plate are visible but obscure. The existence of the two lateral accessory plates shown in the preceding figure cannot be determined, and the apparent jointing of the median appendage of the thoracic plate is produced by the impress of the articulating surfaces of the body segments.
The accompanying figure will illustrate this specimen and is given for the purpose of comparison with the figures upon Plates IV, V and VI of this paper. So far as evidence from this specimen goes there are no means of separating it from Eurypterus proper, the post lateral processes of the articulations, which become spiniform below, being only a greater development of corresponding parts in forms of that genus.
In comparison with the specimens from the coal measures of Pennsylvania, the Illinois species is more robust, the
