202 



SILURIAN TRILOBITES. 



species, I must restrict the name to the fossil first figured. That was a crushed speci- 

 men ; but the finest possible examples have since been obtained from the same locality ; 

 and my friend Mr. J. E. Lee, of Caerleon, possesses the best, — our figured specimen. 

 Except the Barr Trilobite, which it much resembles, we have no species of equal bulk in 

 Britain. It is a Bala Limestone fossil ; the Grug and Birds Hill quarries being in that rock. 



A large and extremely convex species, generally about six or seven inches long. The 

 form a long ellipse, the length being to the breadth as 11 : 5. Head about equal to the 

 thorax, or slightly longer, and the tail a little shorter than either. The head is truly a 

 quarter of a sphere, with square head-angles, and very slightly trilobed by shallow axal 

 furrows, which converge but little, are sigmoid, and reach to the top of the eyes.^ The 

 glabella thus marked out is not wider than the cheeks. The eyes are large for Illanus 

 proper ; narrow, with no furrow beneath them, and near to the axis — scarcely a third of 

 the glabella's width away from it. The cheeks steeply decline towards the eyes, and are 

 thence nearly vertical to the blunt margin. The angles, though called square in the 

 diagnosis, are more acute than a right angle, and the corner is rounded off; still the 

 aspect is unusually square for the genus. The shape of the free cheek, from the very 

 outward curve of the facial suture above the eye, is a trapezoid; the side nearest the 

 eye being about one half that opposite to it— viz., the margin. 



The convexity of the head is such that a line taken transverse to the eyes would be a 

 semicircle, and that from vertex to front a quarter of a circle. On the underside, the 

 coarsely striate^ rostral shield (epistome) is as wide as the glabella, and is itself about 

 twice as wide as long. It is narrowed on either side, more so than in 11. Boiomanni, 

 and abruptly broader in the middle. And in this form it differs again from the more 

 oval shield of //. [Bumastus) Barriensis, between which and this species there are many 

 points of resemblance that strike the eye at first glance. Indeed, 11. Murchisoni leads 

 from the true lllani to that subgenus, as may be seen by what follows. 



The trilobation of the thorax is not very distinct, the axis being almost as broad 

 as that of Bumastus, but much more pronounced than in that subgenus. 



The thorax of ten rings is about five sixths the length of the head. The greatest 

 breadth of the axis is in the seventh and eighth body-rings, where it is double that of 

 the pleurae ; in the first ring, as the axis is spindle-shaped, the breadth to that of the 

 corresponding pleura is 4 : 3 ; in the last ring it is 3 : 2. The fulcrum is placed at one 

 third in the first ring, and at one half in the eighth, ninth, and tenth. The pleurae, flat 

 as far as the approximate fulcrum,^ thence slightly bent down, and inclining backwards. 



1 Within the crust, on the cast, they show strongly the glandular ? depressions opposite the eye, as in 

 Bumastus Barriensis, but not so large. These impressions are as yet very problematical. Can they be 

 muscular attachments ? I think not. 



2 The striae are coarsely imbricato-striate, as in Barriensis. They are parallel in front with the 

 anterior edge, and behind with the posterior edge ; in the middle, nearly direct across. 



3 When the axis is broad, the fulcrum must be always approximate (for mechanical reasons connected 

 with the rolling up), except in very flat Trilobites. 



