798 THE PALEONTOLOGY OF MINNESOTA. 



[Oncoceras exlguitm. 



tinctly moniliform. The external surface of the shell is covered with very line, 

 slightly undulating concentric lines. 



Formation and locality. The most perfect of the single specimens, is in a block of buff limestone 

 of Trenton age, but without precise locality. In association with it are Orthis Jlabellites Hall, O. testudi- 

 naria Dalman, and Plectambonites sericea Sowerby. (Collection of Dr. Robbing). Others are from the 

 lower blue beds of the Trenton limestone, at Janesville and near Bcloit, Wisconsin. 



Genus ONCOCERAS, Hall, 1847. 

 ONOOCERAS EXIGUUM Billings, 1860. 



PLATE LVIII, FIGS. 10 and 11. 



Cyrtoceras exiguum BILLINGS, 1860. Canadian Naturalist and Geologist, vol. v., no. 3, p. 172, 

 flgs. 1718. 



Shell small, short, gently arcuate, gradually expanding toward the aperture 

 and somewhat abruptly constricted. Air-chambers relatively deep, septa evenly 

 convex, with regular sutures and central sipho. Exterior smooth. 



Of six incomplete examples of this little species, some show that the body- 

 chamber occupied from one-half to one-third the length of the shell. Probably 

 none of the shells were more than 30 mm. in length when entire, and the depth of 

 the air-chambers is from 1| to 2 mm. The species is distinguished by its small size, 

 distant septa and gradual inflation. 



F+mation and locality. In the Galena shales near Fountain, Minn. The original specimens were 

 from the Trenton limestone near L'Orignal, Canada. 



Museum Register, No. 8281. 



ONCOCERAS MINNESOTENSE, sp. nov. 



PLATE LVI11. KIUS. 1(1-18. 



Shell moderately large, rapidly expanding, very faintly arcuate, cross-section 

 strictly oval, the major or dorso-ventral, and minor or lateral axes being as 3 to 2i. 

 Septa concave, much more so dorso-ventrally than laterally. Air-chambers moder- 

 ately deep, there being about eight in a distance of 32 mm. The longest example 

 observed has fifteen air-chambers in a length of 45 mm. Sutures regular, with 

 broad, evenly convex lateral, and a rather broad dorsal saddle. The ventral saddle 

 is much the narrower and subacute, the summit of its angle higher than that of the 

 dorsal saddle. Sipho ventral, submarginal, large and moniliform. The siphonal 

 beads are large subrhombic chambers (in section), with thin walls. The opening of 

 the sipho through the septa has about one-half of the diameter of the beads. The 

 siphonal margins of the septa are distinctly calloused. The diameter of the beads 

 equals about one-sixth of the major axis of the septum. Many specimens show 



