352 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



median lobe of each thoracic segment and apparently on the occipital 

 ring of the cranidium. The differences between it and other species 

 are clearly shown by the illustrations. 



The only known dorsal shield preserving cranidium, thorax, and 

 pygidium has a length of 21 mm. The surface is similar to that of 

 B. haydeni. The type specimen was collected by Mr. Sydney Powers 

 of a Harvard University geological party, and is now in the Museum 

 of Comparative Zoology. 



Formation and locality. — Middle Cambrian : Gallatin limestone ; 

 Pole Creek, a branch of Cherry Creek, Gallatin County, Montana. 



BATHYURISCUS (POLIELLA) PRIMUS (Walcott) 

 Plate 46, figs. 6, 6a-d 



Bornemannia prima Walcott, 1908, Canadian Alpine Jour., Vol. i, No. 2, 



p. 241. (Listed.) 

 Bornemannia prima Walcott, 1908, Smithsonian Misc. Coll., Vol. 53, pp. 213, 



214. (Lists species in geological section under new generic name 



Bornemannia. This was prior to the present study of the genus Bathyu- 



riscus.) 



The description of B. haydeni applies to this species fairly well until 

 the pygidium is considered. The pygidium of B. haydeni (pi. 46, 

 figs. 2, 2h) is much larger in proportion to the thorax and cephalon 

 and it also varies in outline, number of segments, length of axial lobe, 

 and width of pleural lobes. The short, relatively small pygidium of 

 B. (Poliella) primus is similar to that of B. (Poliella) anteros (pi. 46, 

 fig. 5) and B. (Poliella) occidentalis (pi. 46, fig. 3). 



Three specimens referred to B. (Poliella) primus have nine seg- 

 ments (fig. 6b) and six have eight thoracic segments each. 



The occipital segment rises and extends slightly backward at the 

 center, where the base of a small spine is shown on several specimens. 

 A similar spine also occurs at the center of the axial lobe of each 

 thoracic segment. 



There is considerable variation in the depth and size of the postero- 

 lateral limb of the cranidium in associated specimens and from more 

 or less widely separated localities. The two extremes of this character 

 are shown in figures 6, 6a and 6b, the postero-lateral limb of the 

 latter being much deeper and larger, which is the case in the greater 

 number of specimens. 



The largest dorsal shield in the collection has a length of 27 mm. 



Bathyuriscus (Poliella) primus is the oldest known species of the 

 subgenus. It occurs at several horizons of the Mount Whyte forma- 



