NO. 24 PTARMIGANIA STRATA RESSER 21 



/. pannula. As Walcott pointed out, this species is characterized by 

 the strong development of the surface pattern. 



Locality 54s. 



Lectotype and paratypes. — U.S.N.M. No. 51468 (except b). 



ACROTRETIDAE Schuchert 



ACROTHELE Linnarsson, 1876 



ACROTHELE ARTEMIS Walcott 



Plate 1, figs. 19-22 



Acrothele artemis Walcott, Smithsonian Misc. Coll., vol. 53, No. 3, p. 82, pi. 8, 

 fig. 10, 1908; U. S. Geol. Surv. Mon. 51, p. 634, text fig. 54, 1912. 



Walcott figured only one shell, a dorsal valve which he called a 

 ventral valve. A ventral valve is here figured, and it shows that the 

 apex is some distance from the margin. 



Locality 54s. 



Holotype and plcsiotypes. — U.S.N.M. No. 51969. 



ACROTHELE PARILIS, n. sp. 



Plate 1, figs. 23-25 



Acrothele subsidna Walcott (part), U. S. Geol. Surv. Mon. 51, p. 656, pi. 60, 

 figs, id, ig, ie, 1912. 



Walcott confused this beautiful species with A. subsidiia, which 

 characterizes the Wheeler shale of the House Range. This happened 

 because he failed to observe the exterior of the ventral valve. 



A. parilis is a large, very beautifully marked form. The ventral 

 valve is almost circular with the elevated apex nearly in the middle of 

 the shell. The dorsal valve is less circular in outline, and in the 

 specimen illustrated is slightly depressed in the middle. The 

 surface is marked by concentric striae. 



Compared to A. artemis, this species is larger and more circular in 

 shape and the apex is nearer the center. 



Locality 54s. 



Cotypes.— U.S.N.M. No. 52014. 



ACROTRETA Kutorga, 1848 

 ACROTRETA SULCATA Walcott 



Plate 1, figs. 11-14; plate 2, fig. 4 



Acrotreta idahociisis sulcata Walcott, Proc. U. S. Nat. Mus., vol. 25, p. 588, 

 1902; U. S. Geol. Surv. Mon. 51, p. 690, pi. 65, fig. 5, 1912. 



This species must be confined to the specimens from the type 

 locality, which is on the strike, but south of, the place where Walcott 



