Book Excerpt, 'Oncoceras Abruptum'

Hall Geol. Rep ^Wis. 1861, p 44

44

ONCOCERAS ABRUPTUM, (n. s.)

Description. Shell small, gradually expanding from the
outer chamber, and contracted again at the aperture. Very
little curved, transversely round-oval, the greatest diameter
being in a dorso-ventral direction, the two diameters being as
seven and eight. Septa but little concave, not very distant,
there being nine in the length of three-fourths of an inch from
the outer chamber, counting on the side. Siphuncle dorsal.

Surface marked by longitudinal ridges, the remains of which
are preserved on the cast.

This description is drawn from two fragments, one of which
is nearly an inch and a half long, retaining eleven of the septa
and a portion of the outer chamber; but the abrupt expansion
of the shell, together with other characters, are sufficient to
distinguish it from any described species. The transverse
diameter of one fragment, where broken off, at the smaller end
is seven-sixteenths of an inch, and at a distance of three-
fourths of an inch it has increased to a diameter of seven-
eights of an inch.

From the O. constrictum, of the Trenton limestone of New
York, it differs in its more closely arranged septa, which are
not arched forward on the dorsum as in that species, and also
in its greater proportional transverse diameter.

Geological Formation and Locality. In the Trenton limestone
group, at Platteville, and in the same position at Beloit, Wis-
consin.