66 SILURIAN TRILOBITES. 



cesses {a, a) rising from the ends of the basal or front margin, and directed obliquely 

 backwards ; and on the sides {b, b) are two inflated, broadly triangular portions characteristic 

 of the genus. These triangular curved plates give the appearance of thickness on viewing 

 the organ from the side (fig. 13c); but the general surface on the inner side is concave, 

 answering to the great convexity of the outer side. 



Thorax much longer than the head, but narrower, and for most part of it parallel- 

 sided, of eleven gently convex rings, which are very minutely scabrous ; the axis is 

 narrower than the glabella, of nearly equal width all the way down, but scarcely so wide 

 as the pleurae. These are linear, and directed straight outwards for two thirds their 

 length, then curved a little backwards, and tapering to a sharp point. The fulcrum, 

 placed at about one third, is of singular structure; a small, semi-oval piece (fig. 14, a) is 

 attached to the posterior edge of each pleura; and against this piece abuts a similar 

 tubercle (b), placed on the front edge of each; and the two pieces, forming together a 

 narrow oval tubercle, are insulated by a deep sulcus from the body of the pleura, which 

 is also constricted and furrowed across at this point so as to have the outer and pointed 

 portion (c) quite distinctly separated from the small inner one. The latter is very 

 strongly divided into two tumid lobes by a short oblique sulcus, and just beyond the con- 

 striction the outer portion rises into a stout boss, giving the tri-tuberculate form cha- 

 racteristic of the genus. The line of the fulcral-points is parallel to the axis for its whole 

 length, and the constriction beneath them, though not very marked on the upper crust 

 (fig. 14), produces a longitudinal ridge on the under surface and a corresponding strong 

 furrow in casts (h'g. I4?d). 



Tail, in all our specimens, very much narrower than the body, with three strong 

 spinous lateral lobes on each side directed backwards, the outer ones a little di- 

 vergent and longest ; all extend equally backwards ; the tail is therefore truncate, but 

 exclusive of the spines, it is broad-triangular, following somewhat the shape of the axis ; 

 it is marked on each side by four short, deep puncta or furrows, which do not run to the 

 margin, even in young individuals. The axis is convex and short-conical, of three distinct 

 ribs, and a small terminal piece, the last very obscurely indicated ; there is, between the lowest 

 spines in the ordinary Wenlock forms, sometimes a blunt, sometimes an acute mucro, and 

 occasionally none at all. 



Variations. — The following have been observed : — In a Dudley specimen the front or 

 forehead-lobe occupies much more than half the length of the glabella, the side lobes 

 being, therefore, more crowded. In another Dudley specimen a large tubercle occurs in 

 the middle of the forehead-lobe. In some individuals the glabella widens more above; in 

 others it is nearly parallel-sided, and the lateral furrows vary in length. The head-spines 

 occasionally reach the third thorax-segment. The margin of the cheek in one specimen 

 is notched at the facial suture. The axis of the thorax is sometimes, though rarely, 



never yet occurred to our observation, nor apparently to others. Yet it seems not unlikely that two 

 plates, an upper and under lip, should form the channel to the mouth. 



