NIAGARA GROITP. 



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 Dal- 

 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 Phucops {Asaphus) limulurvs, 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 kausmanii), 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 somewhat 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 pygidlum, 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, how"ever, 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 trilobile in the whole series, except P. 

 macrophthalmus {P. hufo, 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 



