﻿94 



SILURIAN TRILOBITES. 



the 'Silurian' System, being 4 inches by 2\ broad. Swedish specimens sometimes reach 5 

 inches. Form ovate oblong, not much pointed in front, and obtuse behind. The head occupies 

 more than a fourth the whole length, and is semicircular, — our fig. 8 giving the true shape, 

 while fig. 7 is more pointed than usual. The glabella is more than equal to the width of 

 the cheeks ; and reaches quite to the thickened front margin. It is very convex, especially 

 in front, and is bell-shaped, the base expanded, the large, round, lower lobes fully fth the 

 whole length, the middle ones about half this length, the upper minute. All are convex 

 and well circumscribed. The marginal furrow and the axal furrows deep ; the cheeks 

 convex, but not nearly so much so as the glabella; and the eyes, placed two thirds up, are 

 not very prominent, nor is there any strong buttress connecting them with the central 

 glabella-lobe. The neck- and marginal furrows strong, and all but complete, sometimes 

 (fig. 18) quite so. 



The margin is very thick, and seen on a front view (fig. 1G) not so much bent up- 

 ward as in the last species. The rostral shield is a good deal wider than long, and the 

 sutures converge much toward the labrum, which (fig. 10) is squarish-oblong, with 

 parallel sides beneath the broad ascending processes. The centre is gibbous, with a strong 

 tubercle ; and there is a strong concentric furrow separating a broad margin. The tip is 

 emarginate, with obliquely truncate angular lobes. 



The axis of the 13 body-rings is very convex, and as wide as the deflexed pleurae, 

 from which it is abruptly separated, but not by a furrow. The axis tapers very little 

 backwards. The pleurae, horizontal for a third of their width, bend strongly down at the 

 fulcrum, placed at one third near the head, and in the last joint at |th the width of the 

 pleurae. Pleural groove strong, the forward half of the pleura being the smallest. 



Tail roughly trigonal, with the front greatly arched, and the base-line very little 

 curved. The convex axis occupies one third the width, converges quickly near the tip, 

 and is thence continued by a short appendix to the very end of the tail. It has six or 

 seven rings, and a smooth, convex, terminal portion, beyond which is a smooth appendix. 

 The sides have five strong bifid ribs, very little arched, subparallel, the hinder ones 

 becoming quite longitudinal in direction. The margin is strongly bent inwards at right 

 angles to the surface, but not further incurved. All the tail is granulated. The granu- 

 lation of the margin is very close and fine, (fig. 18$) but over all the rest of the body 

 tubercles are mixed with the granules (fig. a) often conspicuously. 

 FlG - 19 - (See also PI. VIII, figs. 7, 15, &c.) 



The follov.'ing variations may be noted. Our PI. VIII, fig. 7, has a 

 somewhat more pointed head and longer glabella than usual, while fig. 

 16 has it rather shorter than ordinary, and more expanded at the base. 

 Fig. 8 has the normal shape of the glabella-lobes; the basal ones being 

 pyriform and somewhat wider than long. In fig. 14 the length is 

 ^°%h"mene%lulm. greater. Fig. 16 has the large lobes triangular. Fig. 15, in the 

 ^xed granules. 1 " 8 ' Mus. Pract. Geology,' has the granulation stronger than in most 



