396 The American Geologist. June, 1892" 
length from theapex. The tubercle forms a longitudinal medium 
lobe in the anterior 2 of the axis. The tubercle is at the anterior 
margin, slightly elevated above the general convexity of the axis, it 
is less elevated just over the anterior pair of furrows, but behind’ 
this point it rises to twice the hight and terminates abruptly 
at the posterior furrow. The anterior lobes of the axis are dis- 
tinctly separated from the tubercle by a narrow groove; the 
second two are not. Surface apparently smooth, but with indi- 
cations of small wrinkles which unite with each other, so as to 
give a reticulated aspect, similar to that of A. reticulatus Ang. 
The species differs from A. tardus Barr, in having a shorter axis 
in the pygidium, and in having the node of uniform hight its 
whole length. 
AGNostus FABIUS Billings, 1865. Plate 1x, fig. 10. 
Diagnosis.—Head semi-elliptical, uniformly and moderately” 
convex. Limb very narrow. Glabella scarcely elevated above the 
general surface, not defined in front; obscurely so in the posterior 
half by the dorsal furrows, which are parallel, and disappear 
about the middle of the head. Basal lobes small, triangular. 
Pygidium slightly more elongated than the head. Limb narrow, 
flat. Axis about 4 the whole width; it has two pairs of transverse 
furrows, the posterior reaching the median line, where there is a 
small rounded tubercle, located slightly behind the mid-length of 
the axis. The anterior furrows are half way between the tuber- 
cle and the front margin, their inner extremities separated by 
about one-third the width of the axis. 
STRIATION OF ROCKS BY RIVER ICE.* 
J. E. Topp, Vermillion, 8. Dak. 
Sixty years ago the striation of rocks was commonly ascribed. 
to the action of floating ice. Forthe past twenty-five years these 
same markings have as generally been referred to the action of 
land-ice. So much is this the case now, that careful discrimina- 
tions seem to be largely neglected. Very few observations have 
been reported, which are clearly referable to glacio-natant action.. 
While it is quite generally admitted that berg-floe, or river-ice 
*Published with approbation of State Geologist of Missouri. This 
paper was read before the Iowa Academy of Sciences, Dec. 29, 1891. 
