NO. 12 SPENCE SHALE AND ITS FAUNA RESSER 5 



is nearly straight. The crenulated ornamentation gives way some- 

 what at the outer margin of old shells to strong growth lines. 



Localities 55c and 54L. 



Cotypcs. — U.S.N.M. nos. 51448, 27444. 



LINGULELLA Salter, 1866 

 LINGULELLA EUCHARIS, n. sp. 



Plate I, figs. 1-3 



The Lingiilclla species in the Spence shale was referred to the 

 Upper Cambrian species, L. desiderata. L. cuchans is of medium 

 size averaging about 3 mm in length. The illustrations show that it 

 averages broader than any of the numerous species referred to L. 

 desiderata. Growth lines are normally defined, and the usual faint 

 ribbing shows where the outer surface of the shell is exfoliated. 



Localities 55c and 55e. 



Co types. — U.S.N.M. nos. 51704, 51826. 



ACROTHELE Linnarsson, 1876 

 ACROTHELE AFFINIS, n. sp. 



Plate I, figs. 19-22 



The Spence shale species of Acrothelc was referred to A. subsidna, 

 but it averages larger and differs further in the more central position 

 of the apex. The ventral valve had considerable elevation. The ex- 

 terior of both valves is marked by strong growth lines. Well-pre- 

 served outer surfaces show characteristic crenulations. The average 

 size of the shells is about 8 mm. 



Localities 55c, 55e, and 20x. 



Cotypes. — U.S.NJVI. no. 52015. 



ACROTRETA Kutorga, 1848 

 ACROTRETA DEFINITA Walcott 



Plate I, figs. 7-10 



Acrotreta definita Walcott, Proc. U. S. Nat. Mus., vol. 25, p. 584, 1902; U. S. 

 Geol. Surv. Mon. 51, p. 683, pi. 64, figs. 2, 3, 1912. 



Locality 55c. 



Holotype and paratypes. — U.S.N.M. no. 35270. 



