342 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



into an elongate postero-lateral limb that is furrowed by a well- 

 defined groove that delimits a rim that is very narrow near the axial 

 lobe and broader towards the facial suture. Free cheeks unknown. 



" Thorax consisting of 9 segments ; axial lobe very narrow, or 

 only about two-thirds as wide as the lateral lobes, tapering gradually 

 backward, and moderately Convex ; lateral lobes flattened, and lower 

 than the axial ; pleurae broadly and deeply furrowed, and having their 

 free ends apparently falcate. 



" Pygidium intermediate between semicircular and semielliptical, 

 its length being about two-thirds its breadth, while its posterior mar- 

 gin is rounded in outline, and its anterior nearly straight across ; 

 mesial lobe as narrow, proportionally, as that of the thorax, convex, 

 tapering very gradually backward, and nearly reaching the posterior 

 border, showing five or six well-defined segments, with space enough 

 for one or two more behind those ; lateral lobes flat, with five or six 

 broadly furrowed segments that extend to, but not upon, a very 

 narrow, slightly thickened and flattened, smooth margin." ^ 



To the above may now be added : A. node or short spine occurs at 

 the center of each segment of the convex axial lobe of the thorax and 

 pygidium. There is an oval swelling on each side of the axial seg- 

 ments of the thorax close to the longitudinal dorsal furrow and a 

 triangular node-like sweUing in the pleural furrow of each segment 

 with the narrow base of the triangle abutting against the dorsal 

 furrow. This peculiar form of swelling is best seen in B. ornatus 

 (pi. 46, figs. 4, 4a). The terminal section of the pygidium is con- 

 tinued down as a low ridge to the slightly incurved posterior margin. 



Surface smooth or finely roughened by very minute irregular ridges 

 or granules. 



Dimensions. — The largest entire specimen has a length of 30 mm. 

 Associated pygidia indicate a length of 40 mm. for some specimens. 



Observations. — The material illustrating B. haydeni is not as good 

 as for some other species, but it is sufficient to show the principal 

 characters and that its pygidium is unhke that of any other species 

 referred to the genus. 



Formation and locality. — Middle Cambrian: (302) Gallatin lime- 

 stone ; east of West Gallatin River, above Gallatin, Gallatin County, 

 Montana. 



* Quoted from Meek's description, p. 483. 



