786 T ] IE PALEONTOLOGY OF MINNESOTA. 



[Ortlioceras hlllneaturn. 



curvature anteriorly at the sides. They are not crossed by vertical surface lines as 

 far as shown by internal and external casts. 



Sutures regularly transverse, each lying at the bottom of one of the horizontal 

 constrictions. These appear to follow the curvature of the constrictions and annu- 

 lations, and it may hence be inferred that the latter, which are slight, are to some 

 extent increased by, if not due to vertical compression of the shell-tube. In a 

 length of 31 mm. there are fifteen annulations. 



Septum moderately deep; position of sipho not known. 



The species is sufficiently distinguished by the character of its annulations, the 

 position of the septa with reference to the former and the absence of a lineate 

 surface ornament. This seems to me to be the same species as that referred to by 

 Prof. Hall as "Orthoceras (species undermined),"* from the Trenton limestone at 

 Middleville, N. Y. 



The length of the original specimen is 67 mm.; its diameter at the lower end, 

 11 mm.; at the upper end, 13 mm. 



Formation and locality. In the Trenton limestone, Cannon Falls, Minnesota. 



ORTHOCERAS BILINEATUM Hall, 1847. 



PLATE XLVIl, FIGS. 20 and 21; PLATE LIV. FIGS. 6 and 7. 



Orthoceras bilineatum HALL. 1847. Palaeontology of New York, vol. i, p. 199., pi. XLIII, figs. 2a-d. 



Shell of rather small size, gradually expanding; cross-section subcircular. 

 Surface for a considerable distance over the apical region, smooth; but concentric 

 annulations gradually develop, those first appearing being very obscure, those 

 succeeding of increasing strength, until they present the aspect of strong, rather 

 oblique or undulating ridges which are not sharply elevated, but become broader 

 and more conspicuous toward the aperture. The constricted interspaces, which are 

 somewhat wider than the annulations, also become broader toward the body- 

 chamber. In one example there are eighteen annulations in a length of 63 mm.; 

 in another fifteen in a length of 50 mm. In a third example the shell is virtually 

 free of annulations for a distance of about 50 mm., and has a diameter of 14 mm. 

 where the annulations are first well developed. The apertural diameter of an 

 average individual is probably not more than 20 mm. with an entire length of 150 

 mm. These estimates are somewhat conjectural but are based upon the best 

 preserved of numerous examples. 



Palm mt-ilojjy of New Yurie, ol. 1, p. 203, pi. *U|i, flg. 8. 



