No.i3y5. CAMBRIAN BRACHIOrODA—WALCOTT. 249 



Dorsal valve gently convex; area low and a little inclined over the 

 hinge line; delthyriuni broad with a narrow chilidiuni. Casts of the 

 interior show a broad, well defined pscudocriiraliuni, and just in 

 advance of it the adductor muscle scars. 



Ohservation.s. — This species recalls at once N. fe-sthxitahy \i>^ ele- 

 vated ventral valve and spinose surface; it differs from it by its trans- 

 versely suboval outline, large umbonal muscle cavity (pseudocruralium) 

 in the dorsal valve, and sharp-crested ribs. N'. {Jamesella) perpanta 

 has the general form and surface characters of this species. A 

 marked difference in appearance is caused by N. alberta occurring 

 in a silicious shaly matrix and H. {Jamesella) perpada as casts in a 

 (juartzitic sandstone and the strong- surface spines of iV! alherta are 

 sparingly represented on the latter. 



Formation avd locaUty.—W\^d\Q Cambrian. Mount Stephen shale, 

 2,000 feet above Olenellus zone, Mount Stephen section. British 

 Columbia. 



The original specimens were from the collection of Dr. Karl Romin- 

 ger. I now have material lielonging to the Geological Survey of Can- 

 ada, United States National Museum, and Mr. Byron E. Walker, of 

 Toronto, Canada. 



NISUSIA FESTINATA Billings. 



Orthisina festinata Billings, Geol. Sur. Canada, Pal. Foss., I, 1861, p. 10, tigs. 



11, 12; Geol. Vermont, II, 1862, p. 949, figs. 350-352; American Jour. Sci., 



2d ser., XXXIII, 1863, p. 105; Geol. Canada, p. 284, fig. 289. 

 Orthisina festinata Walcott, Bull. U. S. Geol. Sur. No. 30, 1886, p. 120, pi. vii, 



figs. 7, 7a, 7b; Tenth Ann. Kept. U. S. Geol. Sur. 1891, p. 613, pi. lxxii, 



figs. 7, 7a, 7b. 

 BiUingsella festinata Hall and Clarke, Pal. N. Y., VIII, 1892, Pt. 1, p. 230. 



General form subquadrate to transversely semioval, with the cardi- 

 nal extremities subacute to obtusely angular. Hinge line straight, 

 usually equal to or greater in length than the width of the body of the 

 shell. Shell substance fibrous. 



Surface with narrow, rounded, radiating ribs, that increase by both 

 bifurcation and interpolation; the ribs are roughened by concentric 

 lines and ridges of growth that arch about the base of strong, acute 

 spines; the spines are located on the ribs, usually just back of a ridge 

 of growth; they are in more or less irregular, concentric rows 

 toward the front of the shell, but on the central portions they may be 

 scattered without any system of arrangement; each spine is larger at 

 the base, tapering rapidly, and curving gradually backward at about 

 one-half its length. 



The ventral valve is more or less elevated at the umbo and apex; in 

 some examples it is subpyramidal, where the height is to the length as 

 3 to 6, and width 9 mm.; in other shells the proportion is 4 to 11, and 

 width 17 mm.; the slopes from the apex to the margins are nearly" 

 straight or slightly convex; the form of the umbo and apex varies from 



