198 



SILURIAN TRILOBITES. 



The species must have been fully two and a half inches long, by nineteen lines wide ; 

 broad ovate, depressed. The thorax is nearly as long as the tail, whicb is wide and very 

 short, and has the angles broadly and strongly truncate. Our smaller specimen has 

 only a very imperfect bead, which is very convex, probably even gibbous about the base 

 of the distinct glabella ; the axal furrows are broad, short, and rather deep : the eyes 

 remote, placed near the posterior margin, apparently close to it, and so far apart as to 

 be more than half the width of the glabella remote from it. Head-angles (probably) 

 obtuse. 



Thorax of ten rings, depressed ; the axis considerably wider than the pleurae, and 

 strongly, not deeply, divided from them, scarcely tapering backwards, but more so in the 

 young than the adult. 



Pleiu'se flat as far as the fulcrum, which is placed far outwards, at about halfway out 

 in all the rings ; in the first rings nearly as far outwards as in the hinder ones, a character 

 by no means common. Beyond the fulcrum the pleurae are abruptly bent downwards, 

 and very much backwards, almost at a right angle to the line of the pleurae ; indeed, 

 more so than in any British species, except, perhaps, //. Tlionisoni. 



Tail quadrate transverse, the posterior margin elliptical, the front edge slightly 

 sinuated by the arch of the axis, — and with the angles so abruptly truncated beyond the 

 fulcrum as to give an oblong instead of a semicircular shape to the tail. The axis is 

 broader than one third of the width of the tail, marked out by short and rather strong 

 sub-parallel furrows, which reach one third down the tail and are then lost, — at least 

 upon the upper surface. Beneath the crust the anal extremity of the axis is prominent, 

 and forms a narrow sulcus in the cast (fig. 3). This reaches nearly to the margin, and 

 probably indents the broad concave fascia ; but our specimen is not quite complete enough 

 to show this. The fascia, however, is slightly convex near the margin, as indicated im- 

 perfectly in our figure by a shallow furrow ; and extends halfway, or nearly so, up the tail, 

 parallel with the upper surface ; above it ends just outside the fulcral point. 



Locality. — Caradoc schists of Desertcreat, Co. Tyrone. (Both specimens in Mus. 

 P. Geology ; they are Portlock's originals.) 



Ill^nus (III. ?) ocularis, Salter. PI. XXIX, figs. 7, 8 (9 ?). 



Ill^nus ocularis, Salter. Decade 2, pi. ii, p. 4, 1849. 



— — Id. Morris's Catal., 2nd edit., p. 110, 1854. 



— — Id. Catal. Mus. Pract. Geol., p. 5, 186.5. 



//. (//. ?) minor, vix unciam latns, lente convexus ; copHe [sohhn adhuc cojnoio) semi- 

 circulato, oculis longis. Cajmt modice convexum, insuper depressum, marline front ali gihho. 

 (ilabdla genis angitstior, sidcis axalihus incvrvatis, Ms dimidium fere capitis efficientibus . 



