382 The American Geologist. June, 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 Lsevigati the limb becomes obso- 

 lete; in the section Limbati it is broad. The thorax has two 

 segments in all the know^n species. The axis is usually well de- 

 veloped in width, whereas the pleurae are often reduced; when 

 the trilobation is distinct, as in A. rex, the thorax shows the 

 same characteristic; when it is faint, as in A. muUs, the triloba- 

 tion is also indistinct. The first segment is subdivided b}' a groove 

 w^hich gives to the pleurae two bands more or less elevated, the 

 anterior band being the larger; the second segment has the 

 pleurae on each side divided into bands of equal width; the 

 points of each pleura? are directed forward. The pygidium con- 

 forms to the head. In certain species, for example A. hilmUatus^ 

 the pygidium is marked with dorsal grooves; but the head show& 

 no trace of these grooves. In A. rex the axis of the p3'gidium 

 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 tlie head has no border, that of 

 the pygidium is augmented as in A. nudis. The t3'pical axis 

 occupies generally f 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 (-1. nathorsH and A. acnJcafus), also extended into a spine 

 in A. gihhns. In A. rex 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 ^4. 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. hjerulji and A p)^i^nicauda 



