NO. II NEW LOWER CAMBRIAN SUBFAUNA 313 



occupying- a little more than one-third of the surface that extends 

 from the apex of the valves to the front margin. The casts of the 

 interior of the valves show the cardinal area, main vascular sinuses, 

 and the form of the visceral area. 



The average size of the valves is from 4 to 6 mm. in diameter. 



As far as can be determined from the casts of the valves which are 

 compressed in the shale the shells were thinner than those of Oholella 

 chromatica Billings,' and had a somewhat different arrangement of 

 the various features of the interior of the valves. The mode of oc- 

 currence is much like that of O. chromatica and O. atlantica, as the 

 valves occur in large numbers on partings of the shale. If specimens 

 can be found in a calcareous deposit, much closer comparison may be 

 made with other species. 



Formation and locality. — Lower Cambrian : (6ik) Mahto forma- 

 tion ; dark, hard siliceous shale ; northeast base of Mumm Peak above 

 Mural Glacier on west side of Hitka Pass, 6 miles (9.6 km.) in a direct 

 line north of summit of Robson Peak and northwest of Yellowhead 

 Pass, in western Alberta, Canada. 



OBOLELLA cf. CHROMATICA Billings 



Plate 52, fig. 2 



Obolella chromatica Billings. 1861. For synonymy see Monogr. U. S. Geol. 

 Survey, Vol. 51, 1912, p. 591. 



This form is represented by casts of the exterior and partial 

 interiors of several valves of a shell that is very much like what O. 

 chromatica might be if preserved in the same siliceous, shaly matrix. 

 The shell is larger than that of Obolella niida (p. 312) as it averages 

 8 mm. in diameter, and the valves are evenly convex and not flattened 

 as is the case with 0. niida. 



Formation and locality. — Lower Cambrian : (6ik) Mahto forma- 

 tion ; dark, hard siliceous shale ; northeast base of Alumm Peak above 

 Mural Glacier on west side of Hitka Pass, 6 miles (9.6 km.) in a 

 direct line north of summit of Robson Peak and northwest of Yellow- 

 head Pass, in western Alberta, Canada. ^ 



HOLMIA ? MACER, new species 



Plate 54, fig. I 

 This species is characterized by its proportionally large cephalon* 

 and narrow thorax, in these respects resembling Holmia kjcntlfi.' 



^ Monogr. U. S. Geol. Survey, Vol. 51, 1912, p. 591, pi. 54, figs, i, ui-i. 

 - Smithsonian Misc. Coll., Vol. 53, No. 6, 1910, pi. 27, fig. 7. 



