78 SILURIAN TRILOBITES. 



Thorax parallel-sided, scarcely tapering backwards, of eleven thick rounded rings; 

 the axis as wide as the sides, and of equal breadth throughout, very convex ; each joint 

 much raised and rounded (see fig. 4). Pleurae horizontal as far as the fulcrum (fig. 6, d), 

 and then abruptly deflexed, and from this point the pleura tapers outwards to a conical 

 blunt point, which, at the extreme tip, is a little bent forwards. The fulcrum is placed at 

 rather less than half way from the axis ; but in the last segment it approaches much nearer, 

 — to about one third (fig. 6,/). Its place is indicated by a protuberance, both on the forward 

 and hinder edge of each segment (fig. 6, </and/); but these swellings are not isolated 

 tubercles, as in Cheirurus, nor are there any oblique or longitndinal furrows on the pleurae, 

 as in that genus, to break up the uniform convex surface of the segment. 



Tail about semicircular, truncate ; the axis conical, its base of two depressed close-set 

 rings, its apex of one long triangular joint, which is separated from the second joint by a 

 deep depression; from thence it is flattened, or even depressed, for some distance, but 

 suddenly rises to an obtuse and elevated tip (fig. 6, g). When seen endwise the tail 

 presents a bent appearance. The sides are composed of three obtuse convex lobes, which 

 scarcely project on the margin ; the upper one follows the bend of the hindermost 

 pleura, the second is less curved, the third parallel to the axis ; all are deflected, so that 

 an end-view of the tail gives an angular outline, very distinctly seen in the Decade figure 

 above quoted. 



The entire surface of the thorax (fig. 10) and tail, like that of the head, is covered 

 with a fine granulation, the grains of equal size throughout. 



Variations. — Our Dudley specimens have the tail somewhat shorter and wider, and 

 the terminal joint of the axis therefore shorter, than those from Bohemia. Irish speci- 

 mens (figs. 14, 15) are more like the foreign ones in this respect. The space between the 

 lower glabella-lobes is least in these Irish specimens, though some of them have it con- 

 siderably wider than the diameter of the lobes ; in a Wexford specimen the space is pro- 

 portionally as wide as in those from Dudley, which often have the lobes as far apart as in 

 Bohemian examples. 



The species we have to compare this with are many. These are : — the S. scabridus of 

 Angelin, which differs but little, and may be identical ; the S. angustifrons, id., which has 

 flat, expanded lobes to the tail; S. deflexus, S. granulatus, S. conformis, and S. WegeVini, 

 of the same author, have incomplete basal lobes; one figured, but not named, by 

 Dr. Bey rich in his second paper (1846), which has the lobes of the tail lengthened out, 

 and the terminal joint of the axis short.* An eighth is that figured in the lower part of 

 our plate (figs. 27, 28), possibly a Cheirurus of the Adinopeltis group; but having such 

 marked basal lobes, I regard it as a Spharexochus for the present. The last I shall notice 

 is from the Lower Silurian Rocks of Thibet, and has been figured from Col. Strachey's 



This ought to receive a name. The genus is too scanty to render it inconvenient. S. Beyrichii 

 would do verv well for it. 



