2 



BRITISH FOSSILS. 



the Dudley limestone. And yet, perhaps, there is no species of 

 trilobite which has been so much misunderstood ; the confusion 

 apparently arising from this circumstance — that it is rarely, if ever, 

 found out of Britain ; although somewhat similar species have been 

 identified with it, both British and foreign. It was named in com- 

 pliment to Mrs. Downing, of Dudley, from whose cabinet the figures 

 in the " Silurian System were drawn. 



Description. — Length from an inch and a half to two inches. 

 The general form long-ovate, the anterior end being considerably 

 broader, and with the axis following the same lines, and regularly 

 tapering from head to tail. The surface is moderately convex, the 

 axis raised above the sides, not separated by deep furrows except 

 in the head, and more convex in the thorax than in the head or 

 tail. The head is somewhat less than a semi-circle, though just 

 twice as long as broad, the general outline being rather triangular, 

 fi:om an indentation in the curved outer margin on each side of the 

 wide glabella ; the front is not produced, but angular. The glabella 

 occupies more than one third the width of the head in front, and 

 tapers but little backwards, having nearly straight and parallel 

 sides ; it rises considerably above the cheeks, but is rather depressed 

 than convex, especially the forehead lobe, which is not at all inflated, 

 but slopes gradually to the narrow front margin, from which it is 

 separated by a shallow furrow. Neck lobe strong, broader than the 

 first basal lobes, which are transverse and linear ; the middle pair 

 are broader than these, and oval, the direction of the first and 

 second rrows determining their shape — the lower furrow curves 

 downwards, and reaches the side of the glabella ; the upper one, 

 which is abbreviated, curves the reverse way ; the upper lateral lobe 

 is tra^nsverse, scarcely triangular, and bounded above by a sigmoid 

 furrow, which rims very obliquely out above the eye. All the furrows 

 stretch equally towards the middle of the glabella, leaving but a 

 narrow space between their ends ; between the upper pair a short 

 longitudinal depression occurs. The lobes are not swelled between 

 the furrows, but the surface is even and the furrows shallov^ (they 

 are, however, sharply defined on the internal cast) ; the neck furrow 

 and basal furrows are strong — the two upper ones very faint.* The 

 cheeks are steeply bent down, their outer margin not distinguished 

 by any furrow, and they slope gradually from the eye, without any 

 ridge or groove beneath the latter ; the neck furrow is continued 

 almost to the posterior angle, which is rounded off and only shows 

 a slight projection (fig. 10, c) in the place of a spine. The facial 



* Memoirs GeoL Survey, vol. ii. p. 1. pi. 5. fig, 2. 



