726 THE PALEONTOLOGY OF MINNESOTA. 



[UalmanlUui aoliates. 



a distinct trace of a sfngle annulation, but none of any vertical lobation, such as 

 that characterizing most of the upper Silurian species. The median rib of the 

 pygidium is bifurcated toward its extremity. Corda* attempted a division of the 

 Brontei on the basis of the simplicity or duplication of this rib, proposing for such 

 species as show a bifurcation the name Dicranactis, and for those in which it is 

 simple, Holomeris. It has, however, become evident that the duplication of this rib 

 is a feature of minor significance, probably marking a degree of development in 

 the individual, and varying in definition even in apparently full grown animals. 

 Barrande observed! that a division of the species of Bronteus might be based upon 

 the number of ribs on the pygidium, which are either six, seven or eight on each 

 side of the median rib. By far the greater number of species possess seven ribs. 

 Bronteus lunatus, in the possession of but six such ribs, is brought at once into com- 

 parison with the only other forms known to have that number, viz., B. /titic<n/<i 

 Wahlenberg, from the lower Silurian of Sweden, and B, hibernicus Portlock, from an 

 equivalent horizon in Ireland (Caradoc-Bala). These two, with B. lunatus, are the 

 only known lower Silurian members of the genus, all from equivalent faunas, and 

 all possessing the same degree of variation from the type of the genus, and, it may 

 be added, showing in this respect an adolescent condition of development, with 

 reference to the more highly annulated normal Bronteus. 



Formation and locality. Trenton limestone, near Spring Valley; Galena limestone, Wykoff. 



Family PHACOPID^E. 



Genus DALMANITES, (Emmrich,) Barrande, 1872. 

 DALMANITES ACHATES Billings, 'I860. 



Dalmanites achates BIU.IM.S, isiio. Caiiad. Nat., vol. v. p. <;.'!, lif,'. 9. 

 Dalmanites achatex HILLINOS, 1803. Geology of Canada, p. 1*7. fl. I*!!. 



A single fragment of the very characteristic pygidium of this species has been 

 observed from the Galena beds at Wykoff, Minn. (Collection of Mr. Scofield). Mr. 

 Billings' original specimen was from the Trenton limestone of the city of Ottawa, 

 and he speaks of it as being of rare occurrence, though at Trenton Falls, N. Y., it is 

 not uncommon. In Mr. Ulrich's collection are a number of heads and tails from a soft 

 calcareous shale of the Hudson River group at Cincinnati. These have the charac- 

 teristic broad curve of the frontal margin of the head, carried to an extreme, and 

 the anterior lobe of the glabella correspondingly broad and short, giving the 

 ccphalon as a whole a much shorter and more quadrate appearance than the New 



I'rmliMtn finer Mini" L, pp. G8, 89. 1847. 



+8yst<-me tjllurleii, vol. i, p. 840. 



