STY GIN A. 



171 



Fig. 39. 



The sub-genus Nileus has not yet, so far as I know, 

 occurred in Britain, although it is so common in Northern 

 Europe that it may reasonably be expected to be found 

 in our own Lower Silurian rocks. I subjoin a figure of 

 the common N. armadillo, Dalman, to show the near 

 approach made to IllcBnus by this peculiar group of 

 Asaphida. The internal lobes of the glabella are well 

 seen in the cast ; the specimen is a fine one, in the British 

 Museum collection. 



Nileus armadillo, Dalraan. 

 Lower Silurian. Russia (Brit. Mus.). 



Stygina, Salter. 



Among the many new and interesting forms of Trilobites described by Major-General 

 Portlock in his work on Londonderry and Tyrone, a small species of Asaphus is recorded 

 from the Lower Silurian of Tyrone, which he named A. latifrons, distinguishing it from 

 some other species by the breadth of front included within the curve of the facial suture. 

 The species is remarkable for the position of the eyes, which are placed so far backwards 

 and inwards as to be close to the base of the small and narrow glabella. This 

 peculiarity of habit is associated with some other characters which will remove the 

 species from Asaphus, The flattened-oval form, long axis to the tail, and the head- 

 spines, much resemble those of Asaphus, from which the nine ungrooved pleurae 

 efiectually distinguish it. In the partial obliteration of the glabella, number of body- 

 rings, and course of the facial suture, it is closely allied to Illanus, from which its 

 habit differs so much ; and there is enough of the under side preserved to show there 

 was no rostral shield, which last is an essential character of lllcenus. The technical 

 generic character may be thus given : 



Body ovate, flattened ; head and caudal shield nearly equal ; body of nine rings, and 

 with narrow axis ; eyes small, placed far backwards and inwards, near the base of 

 the glabella, which is quite distinct above, and much contracted below. Facial suture 

 marginal along a wide space in front, and below the eyes curved outwards and ending 

 on the posterior margin; no rostral shield; labrum convex,, entire ; pleurae without 

 furrows ; tail with a long and partially furrowed axis. 



