CAMBRIAN BRACK lOPODA WALCOTT 57 



horizon which, on the basis of the associated faunas, is correlated 

 with that at which .1/. (/.) nyssa occurs in Montana. It occurs in a 

 fine, hard, dark gray, siHceous shale in association with Hyolithes, 

 Cntaiaiia, and a fragment indicating the free cheek of a trilobite. 



For:mation and Locality. — Lower Cambrian : Siliceous shale of 

 the Lake Louise formation [\\^alcott, 1908a, p. 5], 3,100 feet 

 (944.9 m.) below the summit of the Lower Cambrian, in cliff rising 

 from the southwest shore of Lake Louise, south of Laggan, on the 

 Canadian Pacific Railway, Alberta, Canada. 



MICROMITRA (IPHIDELLA) NYSSA, new species 



Plate 7, Figure 5 



\'entral valve subcircular in outline, with the posterior margin 

 almost transverse ; form depressed conical, with a minute beak in- 

 curving over the pseudodeltidium. The cardinal slope is compressed 

 in all the specimens, but it indicates that there was an imperfectly 

 defined narrow area. Pseudodeltidium, as far as can be determined, 

 broad and short, with its lower margin broadly arched. Dorsal valve 

 slightly convex, beak marginal. No traces of a false area or pseudo- 

 deltidium have been observed. 



Surface marked by concentric strise and lines of growth that are 

 crossed obliquely by two sets of fine elevated lines. The crossing of 

 the latter lines forms minute, shallow, rhomboidal pits, which give to 

 the surface the appearance of a fine network. On the ventral valve 

 the striae cross the pseudodeltidium. Shell substance corneous. 



Observations. — This is one of the largest shells of this genus. 

 The ventral valve has a length of 11 mm. and a width of 13 mm. 

 In form it resembles Micromitra (Paterina) labradorica (Billings) 

 [1861&, p. 6], and in surface characters, M. (Iphidella) ornatcUa 

 (Linnarsson) [1876, p. 25] and some varieties of M. (I.) pannnla 

 (White) [1874, p. 6]. 



Formation and Locality. — Aliddle Cambrian : About 200 feet 

 (61 m.) above the base of the Wolsey shale, on ridge between Gor- 

 don and Young creeks, about half way between Gordon Mountain 

 summit and Cardinal Peak, Ovando quadrangle (U. S. G. S.), 

 Powell County, ^Montana. 



