702 CAMBRIAN BRACHIOPODA. 



north of the creek which flows into Mill Canyon from the west, about 5 miles (8 km.) west-southwest of Liberty and 15 

 miles (24.2 km.) west of Montpelier, Bear Lake County; and (322) limestone (stratigraphic position unknown) about 

 6 miles (9.6 km.) west-southwest of Liberty and 15 miles (24.2 km.) west of Montpelier, Bear Lake County; all in Idaho. 

 (4h) About 375 feet (114.3 m.) above the base of the Cambrian in the limestone interbedded in the Flathead 

 shales of Peale [1893, p. 21], 1 mile (1.6 km.) north of the junction of Bast Gallatin and West Gallatin (Gallatin) rivers, 

 4 miles (6.4 km.) east-northeast of Logan, Threeforks quadrangle (U. S. Geol. Survey), Gallatin County, Montana. 



ACEOTRETA ETJDIS Walcott. 

 Plate LXV>figure 3f ; Plate LXXV, figure 1. 



Acrotreta kutorgai Walcott (in part), 1902, Proc. U. S. Nat. Mus., vol. 25, pp. 589-590. (Described as a new species, 

 but the description included specimens belonging to both Acrotreta kutorgai and A. rudis.) 



Acrotreta rudis Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 95-96, PL IX, fig. 5. (Described and dis- 

 cussed as below as a new species. Fig. 5 is copied in this monograph, PI. LXV, fig. 3f.) 



The specimens illustrating this species are all more or less crushed and flattened on the 

 surface of the fine argillaceous shale in which they are embedded in large numbers, and they 

 are often in the condition of casts of the interior of the valves. This is well shown by the 

 enlargement of a fragment of the shale on which three ventral and three dorsal valves occur 

 (PI. LXXV, fig. 1). 



The ventral valve, as far as can now be determined, was elevated, conical, and with the 

 apex overhanging the false area, so that when the shells were crushed down the posterior side 

 disappeared beneath. A few fragmentary specimens show the false area to have been distinctly 

 defined, of medium height, and marked by a narrow, shallow median furrow extending from 

 the apex to the margin of the valve. Dorsal valve subcircular, slightly transverse, gently 

 convex, and with a minute beak at the posterior margin. The pedicle opening appears to 

 have been of medium size and situated at the apex of the valve. 



Surface marked by lines of growth and very fine concentiTc striae that continue across 

 the false area and furrow. 



The cast of the interior of the ventral valve shows a small but well-defined visceral area 

 in advance of the apex and a short strong main vascular sinus on each side of the area; one 

 cast shows traces of the sinuses nearly to the front margin. The interior of the dorsal valve is 

 marked by a strong median septum or ridge that extends from the front of a small cardinal 

 area forward nearly to the anterior margin in some examples. A large, oval cardinal muscle 

 scar occurs on each side of the median ridge near the posterior border of the valve; the central 

 muscle scars are small, elongate oval and situated on the sides of the median ridge at about 

 the posterior third of the longitudinal axis of the shell; the two anterolateral muscle scars 

 are on the sides of the median ridge a little in advance of the central scai-s. 



This is one of the large species of the genus. A dorsal valve 4 mm. in length has a width 

 of 4.5 mm. The ventral valves average fi-om 4 to 4.5 mm. from the apex to the front margin. 



In size and outline tliis species may be compared with Acrotreta dejjressa (PL LXVI, figs. 

 8, 8a-c) and A. definita (PI. LXIV, figs. 2, 2a-g). It differs from both in its ventral valve 

 being more elongate. All the specimens of A. rudis are so flattened in the shale that com- 

 parison with uncompressed specimens is very difficult. 



Formation and locality.— Middle Cambrian: (101a) Rogersville shale, just above the road in the hill west of the 

 echoolhouse, 3.5 miles {5.6 km.) southwest 'of Rogersville on the road to Melinda Ferry [Keith, 1896a, areal geology sheet]; 

 (102c) first shale south of the Estillville pike, 3.5 miles (5.6 km.) east of Rogersville, Greeneville quadrangle (U. S. 

 Geol. Survey); and (121) Rogersville shale, road just east of Harlan Knob, 4 miles (6.4 km.) northeast of Rogersville 

 [Keith, 1905, areal geology sheet]; all in Hawkins County, Tennessee. 



AcEOTRETA SABRING (Callaway). 

 Plate LXXIII, figures 5, 5a-d. 



Metoptoma sabrinx Callaway, 1874, Quart. Jour. Geol. Soc. London, vol. 30, pt. 1, p. 196. (Name proposed, not 



described.) 

 Oholella sabrinx Callaway, 1877, idem, vol. 33, pt. 1, p. 669, PI. XXIV, fig. 12, (Described as a new species. The 



specimen represented by fig. 12 is redrawn in this monograph, PL LXXIII, fig. 5b.) 



