416 



CAMBEIAN BRACHIOPODA. 



larger and in having a thicker, stronger shell. Some of the shells in the limestone look very 

 much hke Dicellomus politus (Hall) (PI. LII) when they are not compressed. 



Formation and locality. — Upper Cambrian; (327) Arenaceous limestone east of Gold Camp, Caballos Mountains, 

 New Mexico. 



(302d) Limestone 200 yards (183 m.) north of the southwest corner of sec. 18, T. 28 N., K. 113 W., Uinta County, 

 Wyoming. 



(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; (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; (67) sandstone on Tatur 

 Hill, 7 miles (11.2 km.) northwest of Burnet, Burnet County; (69) limestone near Honey Creek, Burnet County; (71) 

 limestone in Cold Creek Canyon, Burnet County; (68y) interbedded sandstone and limestone, Packsaddle Mountain, 

 Llano County; and (70) limestone near Morgans Creek, Burnet County; all in Texas. 



Middle Cambrian: (302x) Sandstones near the head of Powder River, Bighorn Mountains, Wyoming. 



(llj) Basal part of Bonneterre limestone. Mine Lamotte, Madison County, Missouri. 



(11a) Sandstone between Fust and Armstrong creeks, in the southeast corner of the Maynardville quadrangle 

 (U. S. Geol. Survey), Union County, Tennessee. 



(92x) Conasauga ("Coosa' ') shale at Yanceys Bend, Coosa Biver, southeast of Center, Cherokee County, Alabama. 



V/ 



Oboltjs smithi Walcott. 



Text figures 38A-C. 



Obolus smithi Walcott (in part), 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 62^63, PL VII, fig. 9a (not fig. 9, 

 which represents a specimen of Micromitra {Paterina) major). a (Described and discussed as below as a new 

 species. Fig. 9a is copied in this monograph as fig. 38B.) 



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

 valve subcircular, slightly transverse; convexity apparently moderate, judging from the speci- 

 mens as they occur slightly 

 flattened out in the calcareous 

 shales. The shell was rela- 

 tively strong and formed of a 

 number of thin layers or 

 lamellae that, toward the outer 

 edge of the valve, were more 

 numerous and gave a scaly 

 appearance to the margins of 

 the old shells. 



Surface marked by con- 

 centric lines of growth and 

 numerous very fine, slightly 

 irregular, undulating concentric ridges upon which numerous very minute papillae occur, giving 

 the surface, under a strong magnifymg power, the appearance of being minutely granular. 



A ventral valve 6 mm. in length has a width of 6.75 mm. A sHghtly larger dorsal valve 

 7.5 mm. in length has a width of 8 mm. 



As shown in the cast, the area of the ventral valve is very short and divided by a relatively 

 strongly marked, narrow pedicle furrow, the edges of which were elevated slightly above the 

 general plane of the area. The cast of the interior shows that the visceral area was about 

 two-fifths the length of the valve and that the main vascular sinuses extended rather directly 

 forward from the umbo nearly to the front of the shell, separating very gradually and bounding 

 the interior third of the valve. Nothing has been observed of the muscle scars. 



The cast of the dorsal valve shows that it had a very short area that extended well out on 

 the cardinal slopes; that a low central ridge extended a little more than half the length of the 

 shell and was continued by a slight narrow median ridge; the main vascular sinuses extend 

 directly and obliquely forward well toward the front of the shell in about the same relative 

 position as in the ventral valve. The position of the transmedian and anterior lateral muscle 



a See note accompanying the second reference in the synonymy of Micromitra {Paterina) rmjm (p. 351) and the note following fig. 25, p. 351. 



FiGUKE ZS.— Obolus smithi Walcott. A, E.xterior of partly exfoliated ventral valve (U. S. Nat. 



Mus. Cat. No. 51611a). B, Exfoliated dorsal valve, the type specimen (U. S. Nat. Mus. Cat. 



No. SlOllb). C, Partly exfoliated shell showing laminations (U. S. Nat. Mus. Cat. No. 



51611c). 



Figure 38B is copied from Walcott PDOSd, PI. VII, fig. 9a]. The specimens represented are all 

 from Locality 66c, near Helena, Alabama. 



