CAMBRIAN BRACHlOrODA WALCOTT 97 



Formation and LoCxVlity. — ]\Iiddle Cambrian : Reagan forma- 

 tion, limestone at northwest extremity of the Arbuckle ^lountains, 

 about 4 miles (6.44 km.) east of Homer, Woods County, Oklahoma. 



Genus NISUSIA Walcott [1905a, p. 247] 

 NISUSIA RARA, new species 



Plate; 9, Figures 13 and 13a 



The ventral valve of this species has the general form of that of 

 Xisiisia fcstinata (Billings) [1861&, p. 10], except that it has a very 

 strong and deep median sinus and is more transverse ; the delthyriuia 

 is also larger. A', vara occurs at the same stratigraphic horizon as 

 XisHsia alberta (Walcott) [1889, p. 442], but it differs from the 

 latter in having a larger delthyrium and a strong and deep median 

 sinus. 



The surface of N. rara is marked by rounded radiating ribs that 

 increase by interpolation and bifurcation ; small nodes on some of 

 the ribs indicate the presence of spines on the outer surface. A 

 portion of a convex deltidium is shovi^n that has the outer portion 

 "broken away. The specimen represented by figure 13 has a length 

 ■of 8 mm. ; width, 16 mm. 



Formation and Locality. — Middle Cambrian : Spence shale of 

 the Ute formation, about 50 feet (15.2 m.) above the Cambrian 

 ■quartzitic sandstones and 2,750 feet (838.2 m.) below the Upper 

 Cambrian, in Spence Gulch, a ravine running up into Danish Flat 

 from Mill Canyon, about 6 miles (9.65 km.) west-southwest of 

 Liberty and 15 miles (24.14 km.) west of Montpelier, Bear Lake 

 County, Idaho. 



Subgenus JAMESELLA Walcott [igosa. p. 252] 



NISUSIA ? (JAMESELLA ?) KANABENSIS, new species 



This species is represented by a single broken interior cast of a 

 small ventral valve that has a length of 3.5 mm. ; width, about 5 mm. 

 The cast is convex, with the base of a prominent extension that filled 

 the interior of the beak. The surface is finely papillose, indicating 

 that the interior surface w^as finely punctate. The casts of the 

 ribs show them to have been rather sharply rounded and to have 

 increased by bifurcation and interpolation ; the absence of all traces 

 of casts of spine bases on the ribs leads me to refer the species to 

 the subgenus Jamesella. Area shown only by a narrow rim on one 

 side. The delthyrium was probably quite broad. 



