208 



SILURIAN TRILOBITES. 



with head and tail so very convex, that when extended only a small portion of the former 

 is seen in a vertical view ; while the whole length of the thorax, when not rolled up, is 

 two thirds that of the tail. The pointed or rather parabolic head, when viewed directly in 

 its full measure, is 18 lines long, by 22 broad, and the depth 11 lines (much deeper, there- 

 fore, than in //. Barriensis). Of this breadth the glabella, which is strongly marked 

 only in the cast, occupies 14 lines at the base; thence the furrows quickly 

 converge forward to the deep kidney-shaped depression (or gland) above the eye, 

 and thence again diverge to the curious pits ^ near the front of the head. The gla- 

 bella is most gibbous on the vertex, but is not carinate as in the next species. 



Prom the axal furrows the eye-lobe descends rather rapidly to the depressed long 

 lunate eye, which is subtended by a furrow, but by no raised fold of the crust, as in 

 //. Barriensis ; nor does this furrow terminate forward in a deep wide depression, as in 

 the larger species. The eye is placed at less than half its length from its margin ; the 

 facial suture turns sharply out beneath, but cuts the margin under the eye, not beyond it. 

 Above the eye it bends strongly out, and reaches the margin on a line outside the eye. 

 The whole surface is covered by a close sharp lineation which follows the head-margin, 

 and becomes coarse only on the free cheeks, where the puncta are also very strong. 

 The angles are blunt, but rectangular. 



The labriim (figured from a Brit. Museum specimen) is very perfectly preserved. It is 

 wide, triangular, half as wide again as long, straight along the expanded base of 

 attachment, and with nearly straight sides, forming thus a right-angled triangle with a 

 rather acute apex. Its centre is very gibbous, and the convexity declines steeply to 

 the sides, where a broad lateral depression, continuous with the marginal furrow, separates 

 a thickened recurved rim all round : this is broadest on the sides. A pair of compressed 

 tubercles occurs at the lower third : they are transverse-ovate, and more than their 

 own diameter apart. The surface is covered with squamous lines (like those on the 

 body and head) curved strongly, and interlined, like the body-sculpture, with short lines. 



The thorax-rings, regularly arched, are each convex from back to front, and have the 

 axal line coincident with the fulcrum. The pleurae curve forward, and are thickened on the 

 hinder edge, and striated along the facets. 



The tail, 18 lines long and 21 broad, is all but hemispheric in contour, and most 

 convex at the hinder two thirds. It has hneations mixed with puncta in front, and all 

 the hinder part is covered with punctations only. The edge of the tail is acute, not 

 really recurved, but not at all rounded downwards. The incurved caudal fascia is not 

 broad, but very convex beneath, and is covered closely with granular elevated striae. 

 This fascia is continued of the same breadth beneath the pleurae, as in all Trilobites ; 

 and a young specimen in Mr. E. HoUier's cabinet shows that it occupies half the 

 breadth of the pleurae, a very common proportion in the Asaphidee. 



' These pits represent what are frequently seen in Calymme, Trinucleus, and other genera ; they are 

 points of attachment for the alee of the labrum. 



