819 



point. The dorsal furrows extend all round, but they are not deeply 

 impressed. Neck furrow all across. The fixed cheeks all round the 

 glabella are gently convex, or, rather, the glabella seems to be a little sunk 

 into them. In front of the glabella there is a convex slope ending abruptly 

 at the inner edge of ihe concave border. The eyes are semi-circular, 

 about one-third the length of the glabella ; their anterior angles a little in 

 advance of a line drawn across the glabella at the mid-length ; their centres 

 distant from the dorsal furrow about one-third the width of the glabella. 



The pygidium is semicircular, its width a little less than twice its length 

 and varying from moderately to depressed convex ; anterior angles 

 obliquely truncated for about half the width of the side lobes ; sides and 

 posterior margin uniformly curved into a nearly perfect semicircle. The 

 axis is strongly convex, conical, slightly tapering backwards, a little more 

 than half the whole length ; apex obtusely rounded, somewhat abruptly 

 terminated ; four distinctly defined depressed convex rings (besides the 

 half segment at the anterior margin, and a fifth terminal subtriangular 

 segment at the apex, which occupies twice the space of one of the others). 

 The proportional length of the axis varies a little, but in all the specimens 

 it is distinctly segmented, and well defined at the apex. It also varies in 

 width, being in some one-fourth, and in others one-third the greatest 

 breadth of the whole pygidium. The lateral lobes have a small triangular 

 space on each side, next the axis, which is flat or horizontal. It becomes 

 gradually narrower backwards, and terminates in a point at the apex of the 

 axis. Outside of this the whole of the pygidium is gently concave. There 

 are four pairs of ribs in the side lobes. The first pair, in crossing the flat 

 triangular space, are at right angles to the axis. They then turn back- 

 wards, forming an obtuse angle which varies from 120° to about 150°. 

 The other ribs are more inclined backwards, and the last pair are nearly 

 parallel with the axis. The first three pairs have an indistinct pleural 

 groove extending outwards from the axis to the edge of the triangular flat 

 space. The main grooves between the ribs are shallow and concave. 



In several of the large specimens the side lobes are more nearly fiat, and 

 the ribs more obscure, than they are in the smaller ones. 



The surface is not distinctly preserved in the specimens, but it is not 

 tuberculated, apparently it is smooth. One of the specimens of a large 

 pyo-idium exhibits some indications of concentric fissure-like strise. 



This species appears to have attained a length of 4 inches, but the 

 majority of the specimens are fragments of individuals from 2 to 3 inches 

 in length. 



This species is closely allied to several of those described in the forego- 

 ing pages. The difl'erences are as follows : 



1. B. marginatas. — The head of this species, in the only specimen col- 



