49 



large individual two and a half inches, with a transverse diam- 

 eter at the aperture of three-tenths of an inch. 



The specimens of this species may have been circular when 

 living, as the prolongation of the margin of the aperture would 

 indicate, this not always having the same'relative position, and 

 the greatest extension being sometimes half way between the 

 inner and outer angle. In these specimens the curvature is 

 not quite as great as in those where it is marginal, which 

 would indicate a tubular shell flattened in a direction oblique to 

 the plane of the curvature. 



Geologioal Formation and Locality. In some clolomitic layers 

 of the Potsdam sandstone, at LaGrange mountain, Minnesota. 



GENUS ILLiENUS, Dalman. 



ILL^ENUS imperator, (n. s.) 



I have fragments of a large Illgenus,!of the age of the Ni- 

 agara group, distinguished chiefly by its broad, flattened caudal 

 shield. 



Description. Caudal shield very broad, nearly flat for the 

 anterior half of its length ; the middle lobe rising less than an 

 eighth of an inch above the lateral lobes: dorsal furrow* form- 

 ing a shallow depression which expands about one-third of the 

 length of the pygidium. Middle lobe of the thorax broadly 

 rounded and very moderately convex; lateral lobes flattened 

 for a space equal to half the width of the central lobe, and 

 thence bending backwards at an angle of about thirty degrees. 



A single imperfect specimen measures across the pygidium 

 four and a half inches, with a length (on the curve) of three 

 inches. The middle lobe of the thorax is one inch and three- 

 fourths in width near the posterior end, and the four posterior 

 segments measure one inch along the middle of the axis. 



Geological Formation and Locality. In limestone of the age 

 of the Niagara group, at Racine, Wisconsin. 



Collector. T. J. Hale. 



ILLiENUS taurus, (n. s.) 



Description. Ovate, distinctly trilobed; the central lobe 

 fully once and a half as wide as the lateral lobes. Head large, 

 gibbous, extremely arched; the dorsal furrows continued but 

 little more than one-fourth its length, distant from each other 

 a little more than one-third the entire width of the head; an- 

 terior margin straight to a point a little beyond the suture line 

 on each side ; cheeks making a little less than one-sixth of the 

 4gr 



