2 



BRITISH FOSSILS. 



numerous lenses,* not quite closely set. Facial suture ending on the 

 posterior margin, at the inner third of the width of the cheek, and in 

 front of the eye turned outwards in a sigmoid curve ; it cuts the front 

 margin heyond the parallel of the eye, and continues exactly along the 

 edge of the shield. 



The hypostome is semicircular at its base, which is moderately broad ; 

 it has a squarish tumid centre surrounded by a strong furrow, and is 

 deeply divided at the apex into two ovate-lanceolate forks, between 

 which the shelly plate turns strongly inwards : there is an oval circum- 

 scribed tubercle at the origin of each, most distinct on the inner 

 surface. 



Thorax of eight moderately arched rings, the axis not strongly 

 marked and as broad as the pleurae ; these latter are furrowed for 

 three-fourths of their length, and are curved down, but scarcely bent 

 backward, at the obscure fulcrum, which in the hinder rings is placed 

 at about one-third away from the axis, but much nearer to it in the 

 forward rings ; the anterior edge of each pleura is sharpened, and its 

 termination square. 



Tail parabolic, its width at the broader end one-fourth greater than 

 the length (it appears nearly equal), regularly convex, not sunk in the 

 middle ; axis flattened forwards and prominent behind, terminating 

 abruptly at four-fifths the length of the tail, and ribbed throughout by 

 about 15 furrows. The sides are furrowed for about half their width 

 by 12 or 13 oblique parallel sulci, which end abruptly at the edge of 

 the broad concave margin ; the uppermost furrow is much stronger 

 than the rest. The incurved lower surface of the tail occupies rather 

 more than the breadth of the smooth upper border, and folds round the 

 end of the axis which indents it. The fine lines on this incurved border 

 are not truly concentric with the edge, but appear to branch from an 

 imaginary line along the middle or most concave part. The space left 

 between the upper and lower borders is shallow. On the upper crust 

 of the tail the wavy lines are interrupted and inosculate ; they arch 

 over the axis with the convexity of the curve forwards : on the lateral 

 lobes the curve is reversed.f The same arrangement is seen on the 

 thorax rings, — at the ends of the pleurae they turn upwards, and fig. 8 

 shows the longitudinal disposition of these lines on their under sides. 

 On the head similar lines occur, but we have not specimens to show it 

 well ; the hypostome has them obliquely on the forks (fig. 4). 



* On a specimen lent by Mr. Day, which presents only the lower half of the eye, 

 the number and arrangement is such, that judging from analogy with A. Powisii, the eye 

 of which is very much like this, there must have been 6000 lenses, or probably more. 



f Burmeister has figured a good example of this arrangement of the sculptural lines on 

 the tail of an Asaphus in his "Zeitung fur Zoologie," &c., 1 Band, 1848, t. 1, f. 19. 



