694 CAMBRIAN BRACHIOPODA. 



Acrotreta microscopica has remained without iUustration for many years. I made a large 

 collection in Burnet County, Texas, in 1884, which was increased by Cooper Curtice in 1890. 

 We found the little shell in great numbers associated with characteristic Upper Cambrian 

 fossils. Ill the Eureka district, Nevada, there is a small species of Acrotreta, which, so far as 

 can be determined from the material available for study, is identical with the A. microscopica as it 

 occurs in the Upper Cambrian limestones of Texas. It has essentially the same range of varia- 

 tion in the form of the ventral valve, and the dorsal valves also appear to be identical. (Com- 

 pare figs. 1 and 2a (the lattsr has been broadened by pressure); figs. Id' and 2'; and the side 

 outlines la and 2c; and lb" and 2b.) The ventral valves have the same outline and convexity. 

 The outlines of the convexity of figures le-g should be more nearly as in figures Ih and 2d and 

 not concavo-convex as shown on the plate. 



Formation and locality. — Upper Cambrian: (67) Sandstone on Tatur Hill, 7 miles (11.2 km.) northwest of 

 Burnet, Burnet County; and (70e) limestone near Morgans Creel;, Burnet County; (69) limestone near Honey Creek, 

 Burnet County; (71) limestone in Cold Creek Canyon, Burnet County; (68 and 68e) limestone in Packsaddle Mountain, 

 Llano County; (14b) limestone on Cold Creek at north end of gorge opposite the north end of Sponge Mountain, 2 miles 

 (3.2 km.) south of the San Saba County line, in Llano County; (14g) limestone 1 mile (1.6 km.) west of Cherokee, San 

 Saba County; and (14e) limestone in Bartlett Hollow, 2 miles (3.2 km.) southeast of the mouth of Falls Creek, Burnet 

 quadrangle (U. S. Geol. Survey), Lampasas County; all in Texas. 



(12p) About 225 feet (69 m.) above the igneous rocks, in the limestones of the Reagan sandstone, at the northwest 

 extremity of the Arbuckle Mountains, about 4 miles (6.4 km.) east of Homer, Carter County; (12n) limestones of the 

 Reagan sandstone (in the section 7 miles (11.2 km.) north of Springer this horizon is about 240 feet (73 m.) above the 

 porphyry contact and 40 feet (12 m.) below the Arbuckle limestone), NW. { sec. 1, T. 2 S., R. 1 E., Ardmore quad- 

 rangle, Carter County; (12k) limestones of the Reagan sandstone (in the section 7 miles (11.2 km.) north of Springer 

 this horizon is about 225 feet (69 m.) above the porphyry contact and 55 feet (17 m.) below the Arbuckle limestone), 

 on the west side of Honey Creek, near the southeast corner of sec. 35, T. 1 S., R. 1 E., Ardmore quadrangle (L^. S. 

 Geol. Survey), Carter County; and (9r and 9t) about 45 feet (14 m.) and 170 feet (52 m.), respectively, above the por- 

 phyry contact in the limestones of the Reagan sandstone, SB. J NE. J sec. 2, T. 4 N., R. 13 W., 15 miles (24.2 km.) 

 northwest of Fort Sill, Comanche County; all in Oklahoma. 



TTpper? Cambrian: (8o) Limestone on the slope of the ridge where the range swings around to the northwest, 2 

 miles (3.2 km.) north of Aurum, Schell Creek Range, White Pine County, Nevada. 



Middle Cambrian: (58) Shaly limestones in upper beds of Secret Canyon shale, east side of New York and 

 Secret canyons. Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



[/' Acrotreta microscopica missouriensis Walcott. 



Plate LXVII, figures 4, 4a-b. 



Acrotreta microscopica missouriensis Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 590. (Described as below as a 

 new variety.) 



The shell is uniform^ larger and the vertical median line of the false area more depressed, 

 incurved, and distinct than those of Acrotreta microscopica (Shumard) and the variety teton- 

 ensis Walcott. Some of the specimens of the ventral valves beautifully illustrate the effects 

 of compression. One shell (PI. LXVII, figs. 4' and 4") is hke that of Acrotreta attenuata Meek 

 and the other (PL LXVII, figs. 4a and 4a') resembles the low form of A. ophirensis Walcott with 

 the apex overhanging the false area. The surface of the shells embedded in the fine-grained 

 sandstone has a pai^illose surface resulting from the indenting of the shell by the grains of sand, 

 whereas those from limestone are marked only by fine concentric striae. 



PoEMATiON AND LOCALITY. — Middle Cambrian: (Ilk) Sandstones of the "Edgewise beds," beneath the Mvins forma- 

 tion, St. Francois County; and (lie) thin-bedded limestones south-southwest of Potosi, Washington county; both in 

 Missouri. 



Acrotreta microscopica tetonensis Walcott. 



\y 'Plate LXVII, figures 3, 3a-d. 



Acrotreta microscopica tetonensis Walcott, 1902, Proc. U. S. Nat. Mus., vol. 25, p. 590. (Characterized as below as 

 a new variety.) 



This neat little shell possesses the characters of Acrotreta microscopica (Shumard) and 

 A. idahoensis Walcott^ but differs from both by the projection of the apex of the ventral valve 



