382 The American Geologist. Tune, 1892 
the limb. The surface of the genal zone is smooth in the section 
Limbati, but, in Longifrontes they show a tendency to striate or 
punctate. The limb around the head is always wider in front and 
narrowed towards the thorax. One can distinguish upon its surface 
an internal groove, and an external ridge forming the contour, 
sometimes the limb is extended into small points on each side, as 
in A. josepha. In the section Leevigati the limb becomes obso- 
lete: in the section Limbati it is broad. The thorax has two 
segments in all the known species. The axis is usually well de- 
veloped in width, whereas the pleurz are often reduced; when 
the trilobation is distinct, asin A. rer, the thorax shows the 
same characteristic; when it is faint, as in A. nudis, the triloba- 
tion is also indistinct. The first segment is subdivided by a groove 
which gives to the pleurze two bands more or less elevated, the 
anterior band being the larger; the second segment has the 
pleuree on each side divided into bands of equal width; the 
points of each pleure are directed forward. The pygidium con- 
forms to the head. In certain species, for example A. bibullatus, 
the pygidium is marked with dorsal grooves; but the head shows 
no trace of these grooves. In A. rex the axis of the pygidium 
reproduces that of the thorax. The lateral lobes form a concen- 
tric zone to the contour sometimes united, but often separated 
behind the axis. The limb surrounding the pygidium is some- 
times extended into points. If the head has no border, that of 
the pygidium is augmented as in A. nudis. The typical axis 
occupies generally ” of the total length. In front of the axis is 
located a small triangular border (genou articulaire). The axis 
has three joints; the center lobe is usually the smallest, but at- 
tains the greatest hight; it carries typically on the medium line a 
node, which sometimes is extended backwards over the third 
joint (A. nathorsti and A, aculeatus), also extended into a spine 
in A. gibbus. In A. vex the middle joint on the axis is divided 
by a groove, into an upper and lower lobe. The anterior joint 
of the axis has an inclination to become separated into three 
lobes; the two side lobes are common, The last joint is heart- 
shaped. In A. rex the last joint is short and rounded; in 
A. cyclopyge it is large and rounded. Sometimes the axis is 
long, A. fallax. The side lobes when the axis is short unite be- 
hind it. In the typical species they are divided by a furrow, 
which often becomes obsolete. In A. kjerulfi and A planicauda 
