GENUS ACIDASPIS. 333 



The pygidium is somewhat semielliptical or parabolic, wider than long ; 

 the axis is short, somewhat semioval, with one or more transverse furrows 

 near the anterior margin, while the terminal portion is marked by two 

 faint longitudinal depressions, which are scarcely defined grooves. The 

 median rib at its origin is about twice as wide as the lateral ones, increas- 

 ing gradually, and below the middle of its length more rapidly, to the 

 margin ; where it is four or five times as wide as at its origin, and entirely 

 simple. There are seven lateral ribs on each side, which are very slightly 

 elevated, and the four anterior ones curve gently forward. 



The specimens are for the most part casts or impressions, so that the 

 entire surface characters cannot be ascertained. The head is imperfect, the 

 occipital ring and cheeks being broken off ; but there are several nearly 

 entire pygidia in the collection. 



The pygidium of this species resembles the Bronteus planus of Cord a, 

 as illustrated by Barrande (Systeme Silurien du centre de la Boheme, 

 pi. 42, f. 34, and pi. 38, f. 3), and it is difficult to point out distinguishing 

 characters. The head associated with these pygidia is more nearly like that 

 of Bronteus tenellus (Barrande, ut sup. pi. 47, figs. 36 and 37) ; but it 

 differs from that in some important particulars. 



This species has the pygidia more elongate than B. niagarensis of 

 New- York (Pal. N. Y. Vol. ii, pi. 70, f. 3, pa. 314). 



Formation and locality. In limestone of the Niagara group at Racine, 

 Wisconsin. 



GENUS ACIDASPIS (Murchison). 

 ACIDASPIS DANAI. 



Acidasjpis danai : Hall, in Catalogue of Fossils, Geol. "Wisconsin, Vol. i, p. 423. 



Head transverse, somewhat quadrangular, about twice as wide as long : 



anterior and antero-lateral border ornamented with nodes and short 



spines. Glabella strongly defined by the dorsal furrows, narrower in 



front than at the base ; distinctly lobed, the middle lobe larger than the 



others, separated by deep strong furrows. A strong slightly diverging 



spine from each side of the base of the glabella, with a strong node or 



short spine in the centre. 



The ocular ridges rise a little in front and one side of the glabella, and 



continue in a nearly direct line to the base of the eyes which are in a 



line with the front of the anterior lobes of the glabella. 



This species more nearly resembles Acidaspis vesiculosus, Beyrich, as 



illustrated by Barrande (pi. 38, f. 13). It differs from that one in being 



more transverse, in the narrower front of the glabella, and in the straight 



instead of curving ocular ridges. We have not ascertained whether this 



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