798 THE PALEONTOLOGY OF MINNESOTA. 
[Oneoceras exiguum. 
tinctly moniliform. The external surface of the shell is covered with very fine, 
slightly undulating concentric lines. 
Formation and locality.—The most perfect of the single specimens, is ina block of buff limestone 
of Trenton age, but without precise locality. In association with it are Orthis flabellites Hall, O. testudi- 
naria Dalman, and Plectambonites sericea Sowerby. (Collection of Dr. Robbins). Others are from the 
lower blue beds of the Trenton limestone, at Janesville and near Beloit, Wisconsin. 
Genus ONCOCERAS, Hall, 1847. 
OncoceRAsS ExIGuuM Billings, 1860. 
PLATE LVIII, FIGS. 10 and 11. 
Cyrtoceras exiguum BILLINGS, 1860. Canadian Naturalist and Geologist, vol. v., no. 3, p. 172, 
figs. 17—18. 
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 14 to 2 mm. The species is distinguished by its small size, 
distant septa and gradual inflation. 
Formation 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 MINNESOTENSH, Sp. N0v. 
PLATE LVILI. FIGS. 16—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 24, 
Septa concave, much more so dorso-ventrally than laterally. Air-chambers moder- 
ately deep, there being about eight in a distance of 82 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 
