NIAGARA GROUP. 
305 
tremities, and the lower is not nucleiform, but continues across the axis as a transverse ridge. In 
addition to these minor differences, the facial suture from the base of the eye extends towards the 
margin in a broad curve, coming out much below a line through the posterior margin of the 
eyes, and far lower than in our species. In the caudal shield, likewise, the upper margin is 
nearly direct, and the outer angles scarcely curved; while the arching form of this portion, 
and the rounded angles, are conspicuous in our specimens, and are shown in the figure of Day¬ 
man, and in A. longicaudatus of Murchison. 
A great difficulty has doubtless arisen from the differences between the cast and the fossil 
with the crust remaining. The Phacops ( Asaphus ) limulurus , P. (A.) caudatus and P. ( A .) 
wetherellii of Green, are undoubtedly one species (as are likewise P. (A.) micrurus and P. 
{A.) pleuroptyx , which’are no other than Phacops hausmanii ), and the one which is here 
described ; and we can only account for this multiplication, by supposing that he possessed 
specimens in variable degrees of preservation. 
In all the Niagara specimens, the slight extension in the centre of the front of the buckler is 
observable, though sometimes so slight as scarcely to attract attention. The front lobe of the 
glabella is somewhat oval, and separated from the other part, except in the centre, by a broad 
defined furrow which is expanded at its junction with the longitudinal furrows ; the two other 
transverse furrows or grooves below this, on either side, scarcely reach the longitudinal furrows, 
and, in specimens somewha't compressed, they appear like narrow, oblong, deep indentations, 
which terminate similarly at both extremities. These features are unlike the same parts in P. 
mucronatus , 
I have shown that the spine at the extremity of the pygidium, which is of variable length, is 
formed at the end of the middle lobe, by the meeting of the thickened reflexed border of the 
lateral lobes. This is hollow for a short distance, and beyond that point it is a solid spine. When 
the crust of the animal separates, this spine separates with it, leaving the caudal extremity obtuse 
and scarcely extending beyond the articulations of the middle lobe. Such specimens are more 
common than others, and are usually regarded as perfect specimens : if, however, the adhering- 
stone has not been mutilated, the impression of the border can be seen surrounding the ex¬ 
tremity ; and if the specimen extends far enough, the impression of the spine may also be seen. 
This is perhaps the most common and abundant trilobite in the whole series, except P. 
macrophthalmus (P. bufo, Green). 
Fig. 1. An individual of the ordinary size of this species, in the Niagara group. 
Fig. 2. A smaller individual. This, like all the young specimens I have seen, is narrowed to¬ 
wards the posterior part of the body, more than older specimens. 
Fig. 3. The separate cephalic shield, destitute of the crust, which is of very common occurrence 
in localities where perfect specimens are not found. 
Fig. 4. The caudal shield preserving the crust, and the extension of the same into a long slender 
spine. 
Fig. 5. A specimen of shale covered with fragments of trilobites and other fossils, among which 
are two caudal shields of this species preserving the crust. 
[Palaeontology — Vol. ii.] 
39 
