47 



creek, in Iowa; at Scales Mound, in Illinois ; and, more rarely, 

 in the beds of which this group have been penetrated in the 

 mining district of Wisconsin. 



ORTHOCERAS plaxoconvexum, (n.s.) 



Description. Shell of medium size, gradually expanding from 

 the apex towards the outer chamber, plano-convex; transverse 

 section semi-circular or sub-triangular, the diameters as five to 

 nine. The convex side is a little depressed on each side of 

 the middle, the opposite side nearly fiat, the edges abruptly 

 rounded. Septa moderately concave, arching upwards on the 

 sides, somewhat closely arranged, about five in half an inch. 

 Siphuncle small, central. A specimen of the outer chamber, 

 apparently of this species, is a little more than two and a half 

 inches in length, and one inch and an eighth in width, the 

 short diameter being half an inch; the septa are about one- 

 tenth of an inch distant. 



Surface unknown. 



Geological Formation and Locality. In the Buff limestone 

 of the Trenton limestone group, at Mineral Point and Beloit, 

 Wisconsin. 



GONIOCERAS occidentales, (n. s.) 



Description. Shell elongate, very compressed, extremely 

 expanded laterally, the upper part with curved outline, beyond 

 the middle the edges are more nearly parallel; the length 

 (when entire,) having been a little less than twice the greatest 

 diameter. Upper and lower surfaces convex, the one twice as 

 convex as the other; the two diameters as one to seven; late- 

 ral expansions very thin. Septa deeply concave, numerous, 

 closely arranged, twelve to the inch in the central lobe ; arching 

 forwards on the sides with a sharp retral curve a little within the 

 margin, and running backwards in a narrow extension to the 

 edge at a point opposite or below their junction with the 

 siphuncle in the central lobe. Siphuncle oblate, of medium 

 size where passing through the septa, expanding in the cham- 

 bers to more than one half the smaller diameter of the shell, 

 somewhat bilobate from a constriction above and below. 



Surface apparently smooth, or with only concentric lines of 

 growth. 



This species differs from G. anceps, of the New York rocks, 

 in the less rapid and irregular lateral expansion from the apex, 

 in being thinner in proportion to the breadth, in the more reg- 



