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SILURIAN TRILOBITES. 



Length occasionally near seven inches, generally three or four ; proportion of length 

 to breadth in some 7 X 5, in others 10 X 8, a considerable variation, to which we must 

 refer by and by. " General form a broad oval, depressed ; the head nearly semicircular, 

 and as long as the thorax ; the tail semi-elliptical and longer than either ; glabella at its 

 base occupying one fourth or less of the width of the head, broader and more convex 

 above, and a little pointed in front ; it extends nearly to the front margin, and is marked 

 a little within each side by a longitudinal depression, which is curved, with the convex 

 side inwards. Along this hollow lie five pits, which represent the glabella-furrows : the 

 basal one is the neck-furrow ; that above is deepest, and placed nearer towards the centre 

 of the glabella ; the other three recede more, — the fourth is opposite the top of the eye, and 

 the uppermost just above it. Eyelid large, semicircular, and not touching the glabella. 

 The facial suture above the eye is curved a little outwards 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 broad and striate, more or less 

 concave, and marked off by a distinct marginal-furrow. Eye placed half-way up the 

 head, smooth; the lentiferous surface broad ; its minute structure is shown in PI. XIV, fig. 7. 

 Head-angles short-spined, the spines not produced below the third body-segment. 

 Labrum broad above, narrowing below, where it is suddenly and obtusely pointed (PI. 

 XV, figs. 2, 3) ; 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. 



" The cheeks appear to be nearly smooth, but the glabella is covered by fine, short, 

 broken, almost microscopic lines. The labrum is strongly and concentrically striate. 



" Thorax with the axis gently convex, variable in proportion, but seldom more than half 

 the width of the pleuree. In the broad form it is not so much as this. The pleuras are 

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

 obtuse bend backward and very little downward ; the pleural groove 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 upwards and meets it. The curved sabre-shaped 

 tips are strongly striated transversely. 



" Tail varying from little more than a semicircle in the broad form, to a half ellipse in 

 the narrow form ; very slightly convex ; the axis gently tapering, ribbed by about thirteen 

 or fourteen furrows, which have a slight angular downward bend in the middle ; its 

 obtuse tip projects abruptly. Lateral furrows twelve, or in some specimens fourteen, at 

 nearly right angles to the axis, angularly bent downwards at the place of the fulcrum, 1 

 interlined all the way by fainter furrows, and leaving but a narrow caudal fascia. The 

 interlining furrows nearly meet the primary ones at the end, and here the ribs are sud- 



1 The tail, consisting of anchylosed segments, preserves traces of nearly all the characters of the 

 thorax ; the strong furrows are analogous to the farrows of the pleurae, the fainter lines between to the 

 joints. This is, of course, universal, but Ogyyia Buchii shows it more clearly than most species. 



