152 G. F. MATTHEW ON CAMBRIAN FAUNAS 
size ; are situated on a line drawn across the head at about the mid-length, distant from 
each other about the length of the head. Surface nearly smooth. In small, perfect speci- 
mens no trace of glabellar furrows can be seen, but in some of the large ones four or five 
obscure furrows are exhibited. The largest specimen seen is six lines in length and 
seven in width. It occurs at Chapel Arm, Trinity Bay.” 
In my examples, which are from Manuel River, the centre piece of the head shield 
is more broadly rounded in form than that represented in the woodcut in “ Palæozoic 
Fossils” (Vol. II. Pt. 1, p. 71. f. 40), especially at the posterior angles. The proportions 
given by Mr. Billings agree with those of the examples from Manuel River, but the eyes 
are narrower and less prominent than shown in his figure. The following additional 
characteristics of this species may be given :— 
The head is flat and the glabella outlined by only a faint dorsal furrow. The 
glabellar furrows are scarcely distinguishable on the outside of the test, but on the inner 
surface are more clearly seen ; there are three principal furrows, of which the posterior is 
the longest, and is strongly arched backward as it approaches the axial line, and the ante- 
rior furrow is very short; but behind the main furrow of the three, there is a faint and 
shorter furrow whose course is more directly towards the axis of the glabella. The spine 
on the occipital ring projects backward far enough to cross the third ring of the thorax. 
The thorax has twelve or more segments ; the second, third and fo rth rings of the 
rachis are narrow, the fifth and sixth are wider, and bear spines similar to that on the 
occipital ring; the rings behind those which carry the spines are broader and flatter than 
the anterior rings. The posterior margins of the pleuræ, especially of the anterior ones, 
have a sigmoid curve corresponding to the posterior margin of the head, and curve 
forward at their extremities ; and each is grooved by a shallow sulcus extending from the 
anterior inner angle about two-thirds, or three-quarters, for a short distance diagonally 
toward the extremity of the pleuræ, and for the remainder nearly parallel to and near the 
anterior margin; but in the pleura behind the spined rings of the rachis, these sulci are 
very faint, and tend at their extremities to run toward the anterior margin of the pleuræ. 
The surface of the test appears smooth, but under the lens is seen to be punctate 
with minute punctures of unequal size; the inner surface of the test is minutely pitted, 
and also has scattered pits of larger size. 
There is a broad and a narrow form in this species. In the narrow form, represented 
in the figure, the glabella is nearly cylindrical, and as broad as long; and the rachis of 
the thorax at the spined rings is about one-quarter narrower than the lobes ; in the broad 
form the glabella is considerably broader than long, and the rachis of the thorax at the 
spined rings is of the same width as the side lobes. 
The length of the centre piece of the head shield from the apex to the point of the 
occipital spine, in the narrow form, is 11 millimetres, the width is 12 mm., and the height 
21 mm. The length of the twelve segments of the thorax is 10 mm. and the width 12mm. 
The whole length of the parts preserved is 18 mm. 
Locality —In the Cambrian gray shales at Manuel River, Conception Bay, Newfound- 
land. 
This species is to be compared with A. ceticephalus rather than with A. difformis of the 
European Cambrian rocks, for, although it has a thickened and protruding anterior limb 
to the fixed checks, in its general form, its eyelobes and its posterior margin, it is akin to 
