396 



Description. — Head oblong semi-oval, rather strongly convex, most 

 elevated at the posterior one-fourth of the length, thence descending with 

 a depressed convex slope in all directions to the sides and front ; margin 

 with a very narrow projecting border. The glabella is elongate oval ; 

 width, one-third that of the whole head ; length, rather more than two- 

 thirds the length of the head. It has two transverse furrows which com- 

 pletely or partially divide it into three segments. The anterior furrow 

 extends all across at one-third, or a little more, of the length from the 

 front. The posterior furrow is interrupted in the middle and is only dis- 

 tinctly seen on each side, penetrating one-third the width, while its position 

 is a httle in advance of the posterior third of the length of the glabella. 

 The space between the two inner extremities of the posterior fuiTows is 

 occupied by a low conical tubercle, with the apex directed backwards. 

 At each side of the glabella at the posterior extremity there is a small 

 triangular lobe. The glabella is defined all round by a very narrow 

 groove, just distinctly visible to the naked eye, and from the apex a similar 

 groove runs straight to the middle of the front margin. The surface is 

 ornamented by from fifteen to twenty irregular, slightly impressed, radiat- 

 ing rugose striag. 



In the pygidium the posterior segment of the median lobe is equal to 

 the two anterior ia size ; and there are no triangular lobes at the anterior 

 margin. The tubercle is well developed, and its backward sloping apex 

 reaches nearly to the posterior furrow. It seems to divide the two anterior 

 segments, so that each has a sub-quadrate lobe on each side. The surface 

 is striated like the head. Two heads and one tail have been found. 



Length of the tail three hues and one-fourth ; of the largest head, three 

 lines, and of the other, two lines and three-foui-ths. The width is about 

 equal to, or a little less than the length. 



The contour appears to be not a regular semi-oval ; the sides and termi- 

 nal margins being only gently convex, and the angles broadly rounded. 



The structure of the tail is similar to that both of A. tardus (Barrande) 

 and A. glabratus (Angelin) ; but in these species the median lobe of the 

 head is smooth, and consists of one plate only, without furrows. 



It may be that the tail above figured belongs to a different species, but 

 even if that should be the case, it is specifically distinct from A. tardus and 

 A. glabratus, for these are both smooth, while ours is striated like A. ex- 

 sculptus (Angelin), and besides the proportions of the parts are sufficiently 

 different to be of specific value, especially when the character of the sur- 

 face is taken into account. A. tardus and A. glabratus both belong to the 

 upper part of the Lower Silurian. A. e.vsculptus to Angelin's Region B, 

 which is the upper division of the Primordial Zone in Sweden, 



Lr Limestone, No. 1. 



