726 THE PALEONTOLOGY OF MINNESOTA. 
|Dalmanites achates. 
a distinct trace of a single 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 observedy 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. laticauda 
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 A. 
Genus DALMANITES, (Emmrich,) Barrande, 1872. 
Datmanites acHates Billings, 1860. 
Dalmanites achates BILLINGS, 1860. Canad. Nat., vol. v, p. 63, fig. 9. 
Dalmanites achates BILLINGS, 1863. Geology of Canada, p. 187, fig. 186. 
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 
cephalon as a whole a much shorter and more quadrate appearance than the New 
*Prodrom einer Monographie der bihmischen Trilobiten, pp. 58, 59. 1847, 
{Systeme Silurien, vol. i, p. 840. 
