CAMBRIAN FOSSILS. 449 



Iphidea sp. undet. 

 PL LX, fig. 6. * 



Dorsal valve semicircular, slightly convex. Hinge line somewhat 

 shorter than the width of the shell below ; nearly straight, the rostral angle 

 about 180°. Beak small, not elevated. Surface ornamentation consists of 

 extremely fine radiating and undulating- concentric striae that can be seen 

 in detail only with a strong magnifying glass. Shell substance horny. 



This form is associated with I. sculptilis, and, judging from external 

 characters, is closely related to it. The surface ornamentation is of the 

 same character, and, in the absence of the ventral A'alve, it is difficult to 

 distinguish any specific characters on which to base a new species, although 

 the shell is much larger than that of typical I. sculptilis. 



Formation and locality: Same as last, for Ipliidea sculptilis. 



ACROTRETA Kutorga. 



ACROTRETA GEMMA BillillgS. 



PI. LXII, figs. 2, 2a-e. 



Acrotreta gemma Billings, 1S65: Pal. Foss., Vol. I, p. 216, figs. 201ft-/. 



Acrotreta subconica Meek, 1873: Sixth Ann. Kept. U. S. Geol. Surv. Terr., p. 463. 



Acrotreta attenuata Meek, 1873 : Ibid., p. 463. 



Acrotreta pyxiMcula White, 1874: Geog. and Geol. Expl. Surv. W. lOOtb Merid.: 



Prelim. Eept., Invert. Foss., p. 9. White, 1875: Ibid., Final Eept., Vol. IV, 



p. 53, PI. Ill, figs. 3a-d. 

 Acrotreta gemma Walcott, 1884: Mon. U. S. Geol. Surv., Vol. VIII, p. 17, PI. I, figs. 



1«, b, d,f; PI. IX, figs. 9, 9«. Walcott, 1886: Bull. U .S. Geol. Surv. Xo. 30, 



p. 98, PI. VIII, figs. 1, 1«, b. Walcott, 1891 : Tenth Ann. Eept, TJ. S. Geol. 



Surv., p. 608, PI. LXVII, figs. 5, 5ft-e. Hall and Clarke, 1892 : Pal. Xew York, 



Vol. VIII, Pt. I, p. 102, figs. 55-57. ( ?) Matthew, 1895 : Trans. Xew York Acad. 



Sci., Vol. XIV, p. 126. 



This species was described and illustrated by me in Bulletin No. 30, 

 Monograph VII, and the Tenth Annual Report, of the United States Geo- 

 logical Survey. It occurs with both the Middle and the Upper Cambrian 

 faunas in the Park. 



Formation and locality: Middle Cambrian, Flathead terrane, ranging 

 from the lowest terrane; Gallatin terrane, upper beds; Crowfoot section, 

 Gallatin Range; spur at southeast head of first branch from head of Gallatin 

 Valley, south side; Yellowstone National Park. 



MON XXXII, PT II 29 



