•726 CAMBEIAN BRACHIOPODA. 



probable that either genus is directly descendant from the other, but I think that both came 

 from an ancestor that Uved in early Lower Cambrian time and that Billingsella is further 

 advanced in its development than Nisusia. 



NiSUSIA ALBERTA (Walcott). 



Plate C, figui-es 3, 3a-d. 



Orthisina alberta Walcott, 1889, Proc. U. S. Nat. Mus., vol. 11, p. 442. (Described as a new species.) 

 Billingsella alberta (Walcott), Schuchert, 1897, Bull. U. S. Geol. Survey No. 87, p. 158. (Merely changes generic 



reference.) 

 Orthisina alberta Walcott, Matthew, 1902, Trans. Roy. Soc. Canada, 2d ser., vol. 8, sec. 4, No. 3, p. 109. (Original 



description, Walcott, 1889c, p. 442, copied.) 

 Nisusia alberta Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 248-249. (Described and discussed essentially 



as below.) 

 Nisusia alberta Walcott, 1908, Canadian Alpine Jour., vol. 1, No. 2, p. 245, PI. I, figs. 4 and 4a. (No text reference. 



Figs. 4 and 4a are copied in this monograph, PI. C, figs. 3c and 3d, respectively.) 



Shell transversely suboval, front broadly rounded; the straight hinge line is shorter than 

 the full width of the valves. Surface of shell with numerous radiating ribs that increase by 

 interpolation; on a shell 19 mm. in width there are four ribs near the front margin in a dis- 

 tance of 3 mm. ; the ribs are rather narrow and sharp crested, the interspaces being wider than 

 the ribs. A cast of the outer surface of a shell in siliceous shales shows numerous strong spines 

 irregularly distributed on the ribs very much as in N. festinata (PI. C, fig. 3c). 



Ventral valve elevated at the umbo and apex in some shells, convex and rounded over 

 toward the area in others; area varying in height in different shells, usually elevated and 

 overhanging the hinge line; it is divided by a strong delthyrium that is covered by a convex 

 deltidium of varying length, arched at its front margin and divided by longitudinal lines into 

 tliree parts. 



Dorsal valve gently convex; area low, and a httle inclined over the hinge line; delthyrium 

 broad with a narrow chihdium. Casts of the interior show a broad, well-defined pseudo- 

 crurahum and just in advance of it the adductor muscle scars. 



Ohservations. — This species recalls at once Nisusia festinata by its elevated ventral valve 

 and spinose surface; it differs from it by its transversely suboval outline, large umbonal muscle 

 cavity (pseudocrurahum) in the dorsal valve and sharp crested ribs. Nisusia {Jamesella) 

 perpasta has the general form and surface characters of tliis species. A marked difference is 

 caused by N. alberta occurring in a siliceous shaly matrix and N. (Jamesella) perpasta as casts 

 in a quartzitic sandstone, and the strong surface spines of N. alberta are sparingly represented 

 on the latter. A shell that appears to be identical with tliis species occurs at about 1,600 

 feet above the base of the ]\iiddle Cambrian terrane in Utah. 



The specific name is derived from Alberta, Canada. 



Formation and locality. — Middle Cambrian: (14s) About 2,300 feet {701 m.) above the Lower Cambrian and 

 2,700 feet {823 m.) below the Upper Cambrian in the Ogygopsis zone of the Stephen formation [Walcott, 1908f, p. 210], at 

 the 'fossil bed," on the northwest slope of Mount Stephen; and (35k) Burgess shale member of the Stephen formation on 

 the west slope of the ridge between Mount Field and Wapta Peak, 1 mile (1.6 km.) northeast of Burgess Pass; both 

 above Field on the Canadian Pacific Railway, British Columbia, Canada. 



Specimens that probably belong with Nisusia alberta occur at the following locality, 

 together with other specimens that are somewhat doubtfully referred to the species : 



Middle Cambrian: (54q) A drift block supposed to have come from a horizon 1,700 feet (518.2 m.) above the 

 Brigham quartzite [\A'alcott, 1908a, p. 8], found near the mouth of Wasatch Canyon, east of Lakeview ranch, 5 miles 

 (8 km.) north of Brigham, Boxelder County, Utah. 



Specimens that appear to represent a variety of Nisusia alberta occur at the following 

 localities : 



Middle Cambrian: (57c and 57k) About 2,500 feet (762 m.) above the Lower Cambrian and 2,475 feet (754 m.) 

 below the Upper Cambrian in the limestone forming 1 of the Stephen formation fV^^alcott, 1908f, p. 209]; and (58z) 

 about 1,875 feet (572 m.) above the Lower Cambrian and 3,100 feet (945 m.) below the Upper Cambrian in the lime- 



