26 PPP. liEPturr ov ruoouESs. james hall. 



''pie exriemity of the mesial appendage of its operculum, 

 "as well as in the possession of two little spatulate supx^le- 

 "mentary pieces (ss). Hence we very strongly suspect 

 " that other characters will be found, when better speci- 

 "niens 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 impor- 

 tance to constitute generic or subgeneric distinction may 

 perhaps admit of different opinion, depending upon the in- 

 terpretation 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 bi- 

 furcate. 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 can- 

 not ])e 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 xnirpose 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 articula- 

 tions, which become spinif orni below^, being only a greater de- 

 velopment 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 



