170 SILURIAN TRILOBITES. 



Our fossil seems to have been about three inches long. The general form is very 

 broad-oval, blunt behind, but angulated in front of the semioval head, which is much 

 longer than the tail, and a little longer than the thorax-rings. In our best specimen it 

 is thirteen lines long by two inches wide, and of this width the hour-glass-shaped glabella 

 occupies more than one third below, contracting to much less than a third midway, 

 opposite the eyes, and again expanding to nearly half above, to form the forehead-lobe. 

 No lateral lobes or furrows inteiTupt the very even contour of the glabella, which is 

 divided from the equally convex cheeks by a rather sharp but not deep fm^ow, ending 

 on the neck-margin in a distinct punctum, not strongly enough shown in the plate. 

 In the extremely narrow and almost obsolete neck-segment there is so close an approach 

 to Isotelus, that I think we may fairly look on the species as hnking together the two 

 subgenera. 



The eyes are placed halfway up the head, and are much raised, not very large, 

 approximate (measured from their outer edges they are as far apart as the length of the 

 head). The cheeks are regularly convex, the angles rather acute than rounded, and 

 above the angles is seen the pit for the reception of the pleurae, so conspicuous in Asaphus' 

 (jigas, PI. XXIV, fig. 6. Portlock notices this as a tubercle, but a tubercle on a cast 

 signifies a pit on the true surface. 



The facial suture curves largely out below the eye to more than half the width of the 

 cheek ; above it keeps within the line of the eye to reach the margin, and then continues 

 parallel and close to it all along the front, scarcely angular even at the point itself. 



The body, of eight rings, and the tail, can only be described from a single specimen 

 (fig. 3), and this presents some differences from the central figure (fig. i), which belongs 

 to /. gigas, though, like fig. 3, it has been labelled as /. iniermedius, Portl., a species 

 which must be entirely obliterated. 



The axis of the thorax is as broad as the pleurae, but more convex than in I. gigas ; 

 the fulcrum is rather close to the axis ; the pleural groove short, faint, and not so strong 

 as in that species. The tail of our specimen (fig. 8) is semicircular, with smooth, gently 

 convex sides, and a narrow raised axis, extending slightly more than two thirds down 

 the tail, and marked interiorly by ten or eleven rings, or rather by a double row of 

 puncta (representing the glands described at p. 52), one on each side of the axis. The 

 sides are furnished with one long and broad depression or upper furroAv, but with none 

 else, the sides being free from ribs, as in all Cryptonymi or Isoteli. The fulcrum is 

 strongly marked, and the facet broad, long, and turned much backwards. Caudal fascia 

 broad, strongly striated. 



Locality. — Caradoc, Tyrone (Mus. P. Geology). 



