26 SILURIAN TRILOBITES. 



labrum is narrower than the glabella, and half its length ; but from the position of its base 

 its tip reaches back as far as the middle pair of glabella-furrows. 



Thorax considerably longer than the head ; the rings not very convex ; the axis of 

 nearly equal breadth with the pleurae. These are traversed by a straight deep groove 

 (fig. 36 g), curved rather abruptly down at the fulcrum, which anteriorly occurs at the 

 inner third of their length, and in the posterior ring does not reach further than one fourth. 

 The anterior edge of each pleura is sharpened or facetted to pass under the preceding one, 

 and the posterior edge is thick. 



Each pleura is bent forward at its end, which is notched somewhat deeply, and on the 

 under side of each, in front of this notch, is placed a tubercle. When the animal was in 

 the act of rolling up, the tubercle served as a buttress to prevent the posterior ring from 

 being pushed too far forward ; the tail, too, has similar tubercles on its anterior edge. 



The under side, cleared out with great labour and patience by Mr. John Gray, of 

 Hagley, shows two or three other interesting points distinctly. 



First. The incurved under portion, which is very narrow in the tail, and not much 

 broader beneath the border of the head. But along the pleurae a wider strip is turned 

 inwards, which is smooth in this genus, and has a straight inner edge ; while it presents 

 on its forward margin the tubercle before mentioned. 



The interior ridges of the axis show distinctly along the axal line as short, transverse 

 ridges, more prominent a good deal on their inner margin, which does not even show, 

 except as a slight depression, on the upper side. This broad ridge does not extend into 

 the tail portion. 



Var. a, vulgaris. PI. II, figs. 17 — 25. 



Figs. 22, 24, show about the ordinary form of the species ; with broad glabella, complete 

 glabella-furrows, and pointed front. The tail-furrows are strong, and the apex pointed. 

 The axis of the body, too, is prominent. 



Varieties. — Fig. 24 shows some tendency in the narrower glabella towards the variety 

 constr -ictus, figured in the upper part of the plate. It has, however, very large eyes, like 

 the var. macrops, but the tail wants the lateral furrows, or rather has them much slighter 

 than usual, and the apex less pointed. The body-axis is narrow and prominent. 



Fig. 23, a very large individual, is more convex than usual, and the eyes are depressed 

 so as not to rise nearly to the level of the glabella. The tail is of the ordinary type. 



Var. /3, macrops. PI. II, figs. 26 — 29. 



The variety macrops is distinguished by the very large prominent eyes : — they occupy 

 a large part of the cheek. I counted in one of them 140 lenses. The interspaces are 

 granulated. Fig. 28, which belongs to this marked variety, has a larger head than usual, 



