STATE GEOLOGIST. 9 



fourth and fifth axial rings. They all terminate upon the inside 

 furrow which outlines the limb. The points of termination of the 

 anterior pleurae are on a line with the fifth axial ring. The entire 

 pygidium is bordered by a convex limb which runs from the fulcral 

 points around it. This limb has an inside furrow which runs be- 

 tween the termination of the axis and the limb, and also an exte- 

 rior furrow. 



The surface of the upper dorsal shell is tuberculated. 



Locality and geological position. The Trenton group, probably 

 at Minneapolis, Minn. Presented to the general museum by 

 Dr. Stoneman. 



This species approaches B. senectus, Billings, which has six 

 axial rings and four pleurae. Our species is, however, much larger, 

 and has a greater member of pleurae. The inside marginal furrow 

 in the former species does not extend all around the pygidium, but 

 ends abruptly at the end of the axis. Billings' species comes from 

 the Potsdam group, and ours from a different geological position. 

 Our species has certain afiinities with B. extans, Hall. The most 

 prominent points of difference between the former and that described 

 by Prof. Hall is that the axis is more elevated, and does not term- 

 inate so abruptly behind in B. Stonemanii. The second dorsal 

 surface of B. extans is marked with fine imbricating lamellose striae, 

 whereas our species is tuberculated. 



There are three species of this genus described, which appear in 

 the Trenton group; viz. B. extans, H.; B. longispinus, Walcott; 

 and B. spiniger, H. Of all the species, with the exception of the 

 last named, the pygidmm is known, and differs from our species. 

 The only part of B. spiniger, H. known to us is the glabella, which 

 as tuberculated ; and there is reason to suppose that our species may 

 be the missing part of B. spiniger; but it is doubtful. 



The geological range of the genus is from the Potsdam to the 

 Trenton group. 



(b) 



THE AGE OF THE SANDROCK AT AUSTIN, MOWER COUNTY. 



On page 360, of the first volume of the final report of the survey, the age of 

 the sandrock at Austin is considered, and Prof. H. S. Williams is referred to as 

 authority for identification of some fossils from that rock. In justice to Prof. 

 Williams the full text of his communication relating to these fragmentary fossils 

 is herewith given. If the horizon of the Austin rock be in the Marcellus shale, 

 the overlying limestones appearing in the Cedar valley near the state boundary, 

 and further south, probably all fall into the Hamilton epoch. 



