2 



BRITISH FOSSILS. 



above is placed near the centre of the glabella, the other three re- 

 cede more, the uppermost coming just above the eye. Eyelid large, 

 semicircular, and not touching the glabella. The facial suture above 

 the eye is curved outwards a little to reach the margin, along which it 

 runs for some distance ; beneath the eye it curves largely outwards, 

 and cuts the posterior edge just within the border. The latter is wide 

 and striate, more or less concave, and marked off by a distinct marginal 

 furrow. Eye placed halfway up the head, smooth, the lentiferous sur- 

 face broad, its minute structure is not yet known. Head angles not 

 produced below the fourth body segment. Hypostome broad above, 

 narrowing below, where it is suddenly and obtusely pointed ; the 

 centre is gently raised, and a furrow runs down each side ; there are 

 two transverse furrows near the apex, with compressed tubercles between 

 them. Thorax with the axis gently convex, variable in proportion, but 

 seldom more than half the width of the pleurae. These are flattened, 

 furrowed along the upper margin as far as the fulcrum, at which there is 

 an obtuse bend (fig. 2, c, c), the furrow diverges from the margin 

 here, and does not quite reach the recurved and pointed extremity ; it 

 becomes widest immediately beneath the fulcrum, and from the lower 

 margin of the pleurae at this point a narrow ridge runs obliquely up- 

 wards, and meets it. Tail varying from little more than a semicircle 

 to a half-ellipse, very gently convex ; the axis gently tapering, ribbed 

 by about 13 or 14 furrows, which have a downward bend in the middle ; 

 its obtuse tip projecting. Lateral furrows at nearly right angles to the 

 axis, PtUgularly bent downwards at the place of the fulcrum,* interlined 

 all the way by fainter furrows, and leaving but a narrow flat margin. 

 Incurved under portion of the tail narrow, closely striate, not indented 

 by the axis ; the upper side has a striate band of the same width, which 

 is indented by every rib of the tail. The inferior striated band is con- 

 tinued (of the same width) along the ends of the pleurae, and the striae 

 run in the same direction, while, on the upper surface, they are trans- 

 verse, and reach further inwards. Numerous fine striae, arched upwards, 

 cover the axis. The cheeks appear to be nearly smooth, but the gla- 

 bella is covered by fine, short, broken, almost microscopic lines. The 

 hypostome is strongly and concentrically striate. 



Variations. — It varies in convexity, proportionate length, and in the 

 width of the axis ; this latter is sometimes, though very rarely, more 

 than half the breadth of the pleurae — generally half, frequently less than 

 half. A pair of tubercles on each segment of the axis is present ; they 

 are distinct and strong in some specimens (fig. 6), and faint in others. 



* The tail, consisting of anchylosed segments, preserves traces of nearly all the cha- 

 racters of the thorax ; the strong furrows are analogous to the furrows of the pleurae, the 

 fainter lines between them to the joints. 



