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SILURIAN TRILOBITES. 



Elongate, broadest in front and regularly tapering backwards, pointed at both ends ; 

 semicylindrical, with strongly deflected sides and flattened back ; the head depressed, truly 

 triangular; the tail regularly convex, and rather rhomboidal than triangular, about as 

 long but not nearly so broad as the head. Young specimens seem to have the same 

 general proportions. We may now consider the details, and they are taken from the 

 magnificent central specimen long known as the ornament of Mr. E. Blackwell's collection, 

 and which we are enabled to figure afresh (the specimen served for Murchison's very 

 accurate figure), together with others in the collections of Mr. E. J. Hollier, jun., of 

 Dudley, of Mr. Ketley, and of Mr. H. W. Edgell, of Sandhurst College. 



The largest specimen known, in Mr. Hollier's cabinet, is not given on the plate ; it 

 measures fully six inches by about three inches broad at the base of the head, which 

 forms nearly a right-angled triangle, and is shaped in front like a pointed gothic arch. 

 Behind it is nearly straight, and the angles are obtuse, but not rounded. 



The glabella is less than half the width of the head, and is four-sided, exactly as long 

 as broad, but narrowed rather suddenly towards the front, and therefore apparently 

 longer than broad. It is distinct all round, and shows faintly a central ridge, and traces 

 of three lateral farrows. It is also slightly emarginate in front, and opposite the notch 

 there is a depression in the rather concave front border, which is less than half as 

 long as the glabella. Cheeks moderately convex, and most so toward the outer angle ; 

 the border distinct, except in advance of the angle, where it is fused with the cheek. 

 Neck-furrow plainly marked, as a narrow sharp line beneath the glabella, and a broad 

 furrow beneath the cheeks. The eye is round, prominent, and placed opposite the 

 middle of the glabella, in advance of the centre of the cheek. All the head is covered 

 by large and small puncta, the former equally spaced over the surface ; and these on the 

 outer margin pass into and are mixed with squamous granules and short lines. The 

 facial suture is very prominent, running close along the front border for a short distance ; 

 it describes a narrower arch than the front of the head to reach the eye, and thence curves 

 largely out in advance of the angle. 



The thorax is half as long again as the head, and is very regularly arched from side 

 to side, scarcely a slight indentation marking the place of the fulcrum, Avhich is in this 

 genus the boundary of the axis. 1 Beyond this fulcral point the rings arch down a little, 

 and then suddenly descend in a vertical line, the broad ends of the segments curving 

 forward and much expanded at their tips, especially in the front rings. The facet is very 

 large, commencing at the fulcrum. The pleural groove, as in all the genus, bisects the 

 pleurae very unequally, the forward half being narrow, the hinder broad; the groove is 

 deep, but narrow, ends abruptly before quite reaching the broad tip, and inwards continues 

 quite across the axal lobe, separating a narrow, flat, articular band in front of each segment, 



1 In nearly all other Trilobite genera the fulcrum is beyond the axis ; in the section Dipleura of 

 Ilomalonotus it coincides with it. Some few other genera have it nearly in the same place : /Eylina, 

 Barrandia, and Remopl en rides are instances of this character. 



