424 MR. F. R. COWPER REED ON THE [Aug. 1 896, 



that I know, particularly in the raised ridge-like ribs, the wide 

 interpleural grooves, and the elongated axis. 



The Keisley pygidium is about twice as wide as it is long. 

 The axis is less than one-third the width of the pygidium at its 

 anterior end, and is triangular in shape. It is convex and more 

 elevated than the lateral lobes, but decreases in height posteriorly. 

 Its length is twice as great as its width at its anterior end. It is 

 composed of three segments : the first two form two prominent 

 rounded rings, separated by a wide deep groove ; the third segment 

 is marked off from the second by a somewhat wider groove of the 

 same depth, and consists of an elongated triangular piece with a 

 pointed posterior end and sides converging at an angle of about 50°. 

 It is as long as the whole anterior part of the axis, and its pointed 

 extremity just touches the concave posterior margin of the pygidium 

 between the third pair of ribs. 



The axal furrows are not deep, but are distinct, particularly along 

 the sides of the terminal segment. 



The lateral lobes are almost flat and horizontally extended ; they 

 are rudely triangular in shape, and have the margins scalloped owing 

 to the projection of the rib-ends. The anterior edge is not so 

 strongly curved back as in Sjph. laciniaius (Lindstr.), nor is the 

 fulcrum so distinct, but the edge is similarly formed by the rounded 

 prominent first rib, which is continued with equal strength to the 

 lateral angle at which it projects with a rounded extremity. The 

 second rib, corresponding to the second axal ring, forms a similar 

 prominent rounded ridge on the lateral lobe, but curves backward 

 rather more strongly and increases slightly in width. It projects, 

 like the first rib, beyond the pygidial margin. 



The wide, shallow, concave depression separating it from the first 

 rib is more than twice as wide as the ribs themselves on the margin. 

 Owing to the projection of the extremities of the first and second 

 ribs, the margin makes a re-entrant angle between them. 



The third pair of ribs are almost parallel to each other, and run 

 straight backwards as low rounded ridges expanding posteriorly. 

 They start from the anterior end of the terminal axal segment, and 

 form rounded projections on the posterior margin of the pygidium. 

 They are separated from the second pair of ribs by a wide shallow 

 groove, rather wider than the ribs themselves, and from the terminal 

 axal segment by a similar groove of rather less width. 



The pygidial margin between the extremities of the second and 

 third pairs of ribs forms a wide rounded re-entrant angle, and 

 between the ends of the third pair of ribs themselves a slight re- 

 entrant curve. In Sph. laciniatus this curve is much narrower and 

 stronger. 



It is probable, as mentioned above, that some of the head-shields 

 called Sph. mirus in the Keisley Limestone belong to this new 

 species, but, in the absence of any distinct features by which to 

 separate them, it is impossible to be certain. The exact agreement, 

 so far as I can see, of the head-shields with those of Sj)h. mirus 

 makes one suspect that the specific difference may be capable of 

 detection only in the pygidia. 



