ACROTRETID^. 701 



Cherry Creek show two relatively small cardinal scars near the posterior margin, and a small 

 subtriangular apical callosity. 



Observations. — This, the oldest species of the genus known to me, has all the essential 

 characters of the type species as far as the available information permits of comparison. It 

 is one of the largest shells of the genus, and is a striking feature of the fauna of the higher 

 portion of the Olenellus zone of central Nevada. The most nearly related species appears to 

 be A. attenuata Meek (PL LXIV, fig. 1). 



Formation and locality. — Middle Cambrian: (7j) Limestones at the north end of the Quinn .Canyon Range, 



1 mile (1.6 km.) northwest of the Italian Ranch foothills, Nye County, Nevada. 



Lower Cambrian: (41) Limestone in the Pioche formation fW'alcott, 1908a, p. 11] on a ridge 2.5 miles (4 km.) 

 northwest of the town of Cherry Creek, White Pine County; and (31a) limestone and interhedded siliceous shales of the 

 Pioche formation [Walcott, 190Sa, p. 11], just above the quartzite on the east side of the anticline, near Pioche, Lincoln 

 County; both in Nevada. 



ACROTRETA PYXIDICULA White. 



Plate LXIX, figures 2, 2a-f , 3, 3a-f . 



Acrotreta pyxidicula White, 1874, U. S. Geog. Surveys W. 100th Mer., Prelim. Rept. Invertebrate Fossils, p. 9. 

 (Described as a new species.) 



Acrotreta pyxidicula White, 1877, U. S. Geog. Sm-veys W. 100th Mer., vol. 4, pt. 1, pp. 53-54, PL III, figs. 3a-d. 

 (Described essentially as in preceding reference. The specimens represented by figs. 3a-b and 3c-d are 

 redrawn in this monograph, PL LXIX, figs. 2 and 2f, respectively.) 



Acrotreta gevima Walcott (in part) [not Billings], 1884, Mon. U. S. Geol. Survey, vol. 8, pp. 17-18, (Specimens 

 representing Acrotreta attenuata, A. curvata, and A. idahoensis alta were included with the specimens representing 

 A. ]}yxidicula when this description of A. gemma was written, but only the last two are known to have been 

 figured: A. curvata, PL I, figs. Id and le, and A. idahoensis alta, PL I, figs, la and lb. The specimens repre- 

 sented on PL IX, figs. 9 and 9a, can not be positively located at this time, but it is probable that they should 

 be referred to A. idahoensis alta.) 



Acrotreta gemma Walcott (in part) [not Billings], 1886, Bull. U. S. Geol. Survey No. 30, pp. 98-99. (Specimens 

 now referred to Acrotreta attenuata and A. primseva were included with the specimens representing A. pyxidicula 

 when this description of A. gemma was written, A. primseva being figm-ed, PL VIII, figs. 1, la-b.) 



Acrotreta gemma Walcott (in part) [not Billings], 1891, Tenth Ann. Rept. U. S. Geol. Survey, p. 608. (Specimens 

 now referred to Acrotreta attenuata, A. curvata, A. idahoensis alta, and A. primseva were included with the speci- 

 mens representing A. pyxidicula in this reference to A. gemma, the last three being figured: A. curvata, PL 

 LXVII, fig. 5b; A. idahoensis alta, PL LXVII, figs. 5 and 5a; and A. primseva, PL LXVII, figs. 5c, 5d, and 5e.) 



Acrotreta gemma Walcott (in part) [not Billings], 1899, Mon. U. S. Geol. Survey, vol. 32, pt. 2, p. 449. (Specimens 

 now referred to Acrcireta attenuata, A. curvata, A. idahoensis alta, and A. primseva were included with the speci- 

 mens representing jI. j;i/a;icKcMfa when this description of A. jrejnmo was written, the last three being figured : A. 

 curvata, PL LXII, fig. 2e; A. idahoensis alta, PL LXII, figs. 2a and 2c; and A. primseva, PL LXII, figs. 2, 2b, 

 and 2d.) 



This is a small and very pretty species in which the interiors of the two valves are fairly 

 well shoAvn by casts. A marked feature of the ventral valve is the very strong vascular sinus 

 (fig. 3f), and of the dorsal valve the thickening of the shell in the posterior portion (fig. 3e). 

 The surface of the shell is marked by fine concentric stride and occasional lines of growth. It is 

 one of the small species of the genus, the average size being from 1.5 to 2 mm. Of the known 

 species it may be compared with A. idahoensis Walcott (PI. LXV) from which it differs in the 

 slighter elevation of its ventral valve and the incised median line on the false area. 



Foemation and locality. — Tipper Cambrian: (313f) Limestone at Schellhourne, Schell Creek Range, White Pine 

 County, Nevada. 



(329d) Limestone near Fish Springs, Fish Spring Range, Juab County, Utah. 



Middle Cambrian: (7j) Limestone at the north end of the Quinn Canyon Range, 1 mile (1.6 km.) northwest of the 

 Italian Ranch foothills, Nye County; and (60) limestone in upper beds of Secret Canyon shale, across the canyon from 

 the dump of the old Richmond mine shaft, Eureka district [Hague, 1892, Atlas], Em'eka County; all in Nevada. 



(30z) About 2,450 feet (746.8 m.) above the Lower Cambrian and 1,950 feet (594.4 m.) below the Upper Cambrian, in 

 the upper part of the limestone forming Ic of the Marjum limestone [Walcott, 1908f, p. 180], in the long cliff about 



2 miles (3.2 km.) southeast of Marjum Pass, House Range [Walcott, 1908f, pis. XIII and XV], Millard County, Utah. 



(5b and 54s) Dark blue-gray Langston limestone [Walcott, 1908f, p. 198], just above the Cambrian quartzitic sand- 

 stones, north side of Twomile Canyon, near its mouth, 2 miles (3.2 km.) southeast of Malade, Oneida County; (59f) 

 limestones immediately underlying the Spence shale member of the Ute limestone [Walcott, 1908a, p. 8], in a saddle 



