AGNOSTUS TRISECTUS. 11 



Glabella large, wide, bile-bed. Anterior lobe large, pentagonal, the base being 

 at right angles to the axis, the sides parallel, while the front is formed by two 

 lines which meet anteriorly at an angle of 90° or more; the posterior lobe im- 

 pressed on each side a little in front of the middle, and raised centrally into a 

 tubercle or keel; basal lobes large, forming each an elongated triangle, which 

 reaches about halfway up the side of the posterior lobe. Cheeks separated by a 

 deep groove in front of the glabella, nearly equal in width throughout except 

 posteriorly, where they are encroached upon by the large basal lobes ; marked by 

 deep furrows which radiate outwards from the glabella to the margin, and are 

 there interlined by other furrows which spring from the margin and extend 

 inwards about halfway towards the glabella; in small, and presumably young, 

 forms the furrows are much less strongly marked, and are sometimes almost 

 obsolete. 



Tail somewhat quadrate, with a wide margin which is produced on each side 

 into a small point. Axis broad, divided into three segments, of which the first 

 two are short and the second narrower than the others. A prominent keel runs 

 through the first two segments, and terminates posteriorly in a prominent over- 

 hanging tubercle : in some specimens the keel seems to show traces of the furrow 

 which separates the two segments ; the third segment forms about two thirds of 

 the axis, is widest near its anterior extremity, and terminates obtusely some 

 distance in front of the margin ; it is divided longitudinally into a median slightly 

 raised portion, which is especially prominent anteriorly, and two lateral portions. 

 Lateral lobes with a somewhat finely reticulate surface, confluent behind the axis, 

 separated from the margin by a wide furrow. 



Head- and tail-shields, 4 — 10 mm. long and wide. 



In the ornamentation of its cheeks A. trisectus presents considerable resem- 

 blance to A. reticulatus, and it is quite possible that imperfect specimens of the one 

 may be mistaken for the other, especially if only the head is present. The much 

 greater width of the glabella, the large triangular basal lobes, together with the 

 fact that the front lobe of the glabella is perfectly defined, will, however, generally 

 suffice to distinguish even an imperfect specimen of A. trisectus. In the case of 

 the tail there is very little difficulty, for in A. trisectus the width of the axis and 

 the bluntness of its termination will usually be apparent even if the longitudinal 

 division should not be visible. 



The very numerous specimens from our various British localities generally have 

 the anterior lobe of the glabella much more obtuse in front than is represented in 

 Tullberg's figure ; and, indeed, the more perfect specimens show it to be distinctly 

 pentagonal in shape. In many cases also the lateral indentations of the posterior 

 lobe are very distinctly marked, almost separating two lateral nodules on each side 

 from the smooth central portion, much as Tullberg has described in the case of 

 J. reticulatus. Lastly, in many of our English specimens the reticulation of the 



