420 CAMBRIAN BRACHIOPODA. 



Obolus willisi (Walcott). 

 Plate XXIII, figures 1, la-j. 



Obolus {Lingulella) willisi Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, pp. 418-419. (Described and discussed 

 as below as a new species.) 



General form broadly ovate, with the ventral valve obtusely acuminate and the dorsal 

 valve almost transversely ovate, the posterior margin being very broadly obtuse ; convexity 

 moderate m the specimens preserved in the calcareous sandstone. Surface of shell marked 

 by concentric lines and strise of growth, with very fine irregular strise between them; a few 

 specimens show very narrow, shghtly irregular, interrupted, radiating ridges or undulations 

 (PI. XXIII, fig. Ij); inner surface of the shell more or less strongly pitted or punctate. This 

 character varies greatly in casts from the same layer of shale, as may be seen by comparing 

 Plate XXIII, figure lb, where it is extreme, with Plate XXIII, figures la, le, and Ih. The 

 shell, as preserved in the argillaceous shale, is relatively thin. 



The largest ventral valve, which is shortened slightly by distortion, has a length of 10 

 mm.; width, 8.5 mm. A well-preserved dorsal valve 8.5 mm. in length has the same width, 

 while another associated dorsal valve 6.5 mm. in length has a width of 7 mm. 



As shown in the casts, the area of the ventral valve is rather long and is divided midway 

 by a strong cast of the pedicle furrow, and again midway between the pedicle furrow and the 

 lateral margins by a well-defined flexure line; fine strise of growth cross the area parallel 

 with its base. The area of the dorsal valve is relatively short and extends far out onto the 

 cardinal slopes; it is marked by clearly defined but not strong flexure lines. Casts of the 

 interior of the ventral valve show traces of the visceral cavity (v) and the main vascular 

 sinuses (vs). In the dorsal valve a narrow median septum is all that is shown, with the excep- 

 tion of faint indications of the central and anterior lateral scars (h and j) (PI. XXIII, fig. Ig) 

 and what appears to be the transmedian (i) (PI. XXIII, fig. le). 



Observations. — ^This species was at first compared with Oiolus larnborni (Meek) (PI. XXII). 

 Like that, it has a strongly pitted or piuictate inner surface, and the ventral valve has the 

 same general outline. The dorsal valve, however, is much more transverse and obtuse, and 

 the central muscle scars in the dorsal valve appear to be somewhat differently located. 



This species has considerable vertical range; some of the specimens associated with the 

 Middle Cambrian fauna m the Conasauga shale are very much hke 0. lamhorni from the 

 RogersviUe shale of Tennessee. 



I take pleasure in naming the species in honor of Mr. Bailey Willis, who for a considerable 

 time had charge of the work in the region where these specimens were collected by Dr. Cooper 

 Curtice. 



Formation and locamty. — Upper Cambrian: (93) Shales and limestones at the base of the Knox dolomite, 

 near Jordan's, just below the ford on Cowan Creek; (93n) shales in the southern part of Jordan's farm, near Jordan's 

 crossroads; and (93x) shales on the Clarke farm, near the ford across Cowan Creek, about 2 miles (3.2 km.) north of 

 Jordan's crossroads; all about 8 miles (12.8 km.) southeast of Center, Cherokee County, Alabama. 



(96b) Shales in a raihoad cut on the Southern Railway, near Cave Spring, 10 miles (16.1 km.) southwest of 

 Rome, Floyd County, Georgia. 



(122) Shale southwest of Town Knobs, 1 mile (1.6 km.) northwest of RogersviUe, Hawkins County, Tennessee. 



Middle Cambrian: (146) Conasauga shale on the Jacksonville road, 8 miles (12.8 km.) east of Center; (91) Cona- 

 sauga shale at Cedar Bluff; (94) shales 0.25 mile (0.4 km.) beyond Steel Ford, Cowan Creek, about 8 miles (12.8 km.) 

 southeast of Center; and (94a) Conasauga shale, in Coosa Valley, east of Center; all in Cherokee County, Alabama. 



(140a) Shales 200 yards (182.9 m.) east of Thomas Mills, 5 miles (8 km.) north of Cave Spring [Hayes, 1902, his- 

 torical geology sheet], Floyd County, Georgia. 



(101b) RogersviUe shale, just east of the schoolhouse, 3.5 miles (5.6 km.) southwest of RogersviUe, on the road 

 to Melinda Ferry [Keith, 1896a, areal geology sheet], Hawkins County; (107a and 107b) shales and sandstone of the 

 Rome formation in railroad cut in Bull Run, northwest of Copper Ridge [Keith, 1896b, areal geology sheet], 11 miles 

 (17.6 km.) northwest of Knoxville, Knox County; (14a) sandstone of the Rome formation along First Creek Gap, 

 4 miles (6.4 km.) north-northeast of Knoxville [Keith, 1905, areal geology sheet], Knox County; (106a) reddish-brown 

 shales in middle of valley east of Shooks Ridge, in Bays Mountains, 10 miles (16.1 km.) southeast of Knoxville, Knox 

 County; (374e) shales collected near the State line, 2.25 miles (3.6 km.) north of Peltier, Estillville quadrangle (U. S. 

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

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



