NO. 5 CAMBRIAN TRILOBITES 33 1 



a small, sharp spine that extends obliquely upward and backward. 

 Fixed cheeks moderately convex, with strong postero-lateral limbs, 

 narrow central section, and small antero-lateral limbs ; a well-defined 

 palpebral ridge curves around the eye and extends obliquely forward 

 across the cheek to the dorsal furrow beside the glabella ; palpebral 

 lobes narrow and varying in length from one-fourth to more than 

 one-half the length of the cephalon. Free cheeks of medium width 

 and terminating posteriorly in a sharp genal spine ; visual surface of 

 eye narrow and elongate. 



Thorax with 8 to 9 segments. (The genotype has nine.) Each 

 segment has a node or spine on the median axis and a very distinct, 

 rather broad pleural furrow that extends nearly to the outer termina- 

 tion of the segment. In the type species, B. haydeni (pi. 46, fig. 2). 

 a narrow, elongate triangular ridge extends from the axis out into 

 the pleural furrow ; this character is strongest in B. ornatus (pi. 46, 

 fig. 4), and traces of it are found in all the species now referred to 

 the genus. The pleural lobes of the segments curve slightly back- 

 ward and terminate in a short falcate point. 



Pygidium semicircular in outline. Median axis nearly as long as 

 the pygidium, convex and marked by several transverse furrows that 

 outline transverse segments ; both furrows and segments extend 

 across the pleural lobe to a narrow border. 



Hypostoma of B. howelli, rounded subtriangular in outline ; central 

 portion oval, convex and separated posteriorly by a shallow curved 

 furrow, from a low transverse tubercle ; a narrow margin merges 

 into rather large posterior wings that form the anterior section of 

 the hypostoma. 



Surface with many very fine, irregular, inosculating ridges that 

 give it a roughened appearance, and when slightly weathered it 

 looks as though there were fine shallow pits thickly scattered over 

 the surface. 



Dimensions. — The largest specimen of the dorsal shield of the 

 type species B. haydeni, has a length of 21 mm. ; B. rotnndatiis 

 reaches 52 mm., while B. ornatus is 21 mm., and B. anax 37 mm. in 

 length. Separated cranidia and pygidia of B. anax indicate that the 

 entire dorsal shield sometimes was 90 mm. or more in length. B. 

 (Poliella) primus, the oldest known species, has a length of 30 mm., 

 and fragments indicate individuals that were 46 mm. long. The 

 largest entire dorsal shield oi B. atossa has a length of 41 mm., but 

 fragments and pygidia indicate that the species sometimes attained 

 a length of 50 mm. or more. 



