150 SILURIAN TRILOBITES. 



and its place is taken in strata of the same age in Sweden by the equally large A. heros, 

 Dalman, a species which Burmeister and others have confounded with it. 



Besides, it belongs to a group of Asaphi, which, by their thinner crust, narrower axis, 

 more expanded form, and strongly furrowed pygidium, conduct easily to Ogygia. This 

 subgenus was first noticed in the 'Memoirs of the Geological Survey,' Decade 2, in 1849, 

 and has been generally adopted. 



Description. — Length of large specimens eleven and a half inches ; breadth more than 

 six inches. General form true ovate, more obtuse in front, moderately convex; the 

 head semicircular, the tail much longer, more convex, and parabolic. Head sculptured, 

 very gently convex, with only a very slight concavity marking off the margin. The 

 cheeks are broad, and their hinder angles produced into short spines, which only reach 

 the third thoracic segment. The glabella is convex in front, broad-clavate, well defined 

 forwards, only slightly so on the sides, and as long as the breadth between the eyes. 

 The forehead-lobe large, round, and rather protuberant, especially in old specimens. 

 Beneath this lobe, and at about the level of the eye, is the uppermost of three short, 

 obscure, oblique lobes, the basal one of which is largest. There is at the base of the 

 glabella a tubercle, and beneath it a shallow transverse impression marking the place 

 of the neck-furrow. Eyes rather large, depressed, much arched, placed behind the 

 middle of the head, and close to the glabella in this species. 



The base of the eye-lobe is constricted, the lentiferous surface probably narrow, and 

 the numerous lenses beneath the smooth cornea are not very closely set (fig. 7). 

 Facial suture curving widely out on the posterior margin to half the width of the cheek, 

 and in front of the eye turned outwards in a sigmoid curve to cut the front margin 

 beyond the parallel of the eye, and continue exactly along the front edge ot the shield. 



The hypostome is yet unknown ; the labrum large, with a semicircular base, which is 

 moderately broad only ; a squarish slightly tumid centre, surrounded by a strong furrow, 

 in which are set obliquely a pair of minute transverse ovate tubercles. The apex is 

 deeply divided into two ovate-lanceolate forks, between which the shelly plate turns 

 strongly inwards ; the length of the lobes is greater than one third of that of the whole 

 labrum. 



Thorax of eight moderately arched rings ; the axis is not strongly marked out, and is as 

 broad as the pleurae ; these latter are grooved for more than half their length, the strong 

 groove bounded behind and before by strong ridges, which form a prominent node at 

 their point of junction beyond the groove ; from thence the pleurae rather flat. They are 

 curved down, but scarcely backward, at the obscure fulcrum, which in the hinder rings 

 is placed one third away from the axis, but nearer to it than in the forward rings. The 

 anterior edge of each pleura is sharpened and coarsely striate, and its termination square. 



Tail parabolic, a little truncate at the end ; its width at the front one fourth greater 

 than the length (though apparently not more than equal to it) ; not quite regularly 

 convex, the axis being flattened in front, narrow, scarcely one fourth the width of the 



