784 THE PALEONTOLOGY OF MINNESOTA. 



[Orthooeras nicolletl. 



species than are here made out. The annulated species are more readily 

 distinguished by their surface variations, but among the smooth forms the exterior 

 of the shell so rarely retains the surface sculpture that little basis remains for the 

 determination of specific traits. No attempt is here made to follow the subgeneric 

 distinctions introduced principally by Hyatt among orthoceran shells, as in the first 

 place, the divisions are based largely upon variation in ornament and contour, and, 

 secondly, our material is not sufficiently complete in its representation of the young 

 stages to justify a subdivision of this kind. 



ORTHOCERAS NICOLLETI, sp. nov. 



PLATE JtC. FIGS. 1-2. L V, 



Tube of moderately large size, very gradually tapering, slightly arcuate in the 

 original specimen, but this appears to be, to some extent, casual. Transverse section 

 circular or subelliptical. Surface covered with strong, distant annulations which 

 are sharp, rather narrow at the base, elevated and quite oblique in their direction, 

 curving downward in traversing the shell from the convex (dorsal?) side to the inner 

 side of the specimen. This obliquity increases very considerably toward the aper- 

 ture. The annulations are separated by broad and deep constrictions whose width 

 increases toward the aperture. Sutures transverse and even; septa regularly and 

 somewhat deeply concave, crossing the shell in such a manner as to transect the 

 annulations and constrictions. The interval between the septa appears to be about 

 the same as that between the annulations, but this is not distinctly shown in the 

 specimen, the suture and septum being clearly displayed only at the lower extrem- 

 ity. The finer surface ornamentation, if such existed, is not retained. In a length 

 of 145 mm. the shell bears eleven annulations, the distance between the first two on 

 the outer or curved side being 11 mm., between the seventh and eighth, 15 mm. on 

 the outer side, and 11 mm. on the inner. The depth of the septum exposed is 7 

 ram. The diameter of the shell at its first annulation is 33 mm., at the last, 36 mm. 



This species is strikingly characterizad by its strong, oblique annulations and 

 slender tube. It is, perhaps, most closely allied to the Orthoceras olorus Hall, but its 

 difference in the features mentioned serve to distinguish it. 



Formation and locality. ID the Trenton limestone at Belle Creek, Minnesota. Collection of W. H. 

 Scofleld. 



ORTHOCERAS ANELLDS Conrad, 1843. 



PLATE flMH. FIGS. 22 2.'). XL. V II 



Orthoceran annellus CONUAD, 1843. Proc. Acad. Nat. Sci. Phila., vol i, p. 334. 



Orthoceras anellum HALL, 1847. Paleontology of New York, vol. i, p. 202, pl.'XLin, figs. 6a-/. 



To this species are assigned two fragments of small conchs characterized by their 

 very gradually expanding sides, sharp, regular, almost imperceptibly arcuate annu- 



