516 CAMBRIAN BRACHIOPODA. 



may be seen by comparing Plate XL VIII, figures 2a, 2e, 2i, and 2k of the ventral valve and 

 figures 2b, 2c, 2d, 2f, and 2g of the dorsal valve. The convexity of the valves is moderate, 

 that of the dorsal valve being a little more than that of the ventral (fig. 2j). 



The surface of the shell is marked by concentric lines and stride of growth, with very fine, 

 concentric striae between them that are sometimes slightly undulating ; on some specimens very 

 faint radiating strife can be seen with a strong lens ; when the outer layer is exfoliated the inner 

 layer is marked by fine radiating and concentric striae in addition to the stronger concentric 

 strise; as far as can be determined from the imperfect casts of the interior the inner surface of 

 the shell was nearly smooth. The shell appears to be formed of a very thin outer layer and 

 one or more thin inner layers or lamellse ; toward the frontal margins the oblique lamellae increase 

 in number, but do not give any considerable thickness to the shell. 



One of the largest of the ventral valves referred without doubt to this species has a length 

 of 7 mm. and a width of 5.5 mm. An associated dorsal valve is shghtly shorter in proportion 

 to the width. The average size is smaller, not exceeding 5 mm. for the length of the ventral 

 valve. One unusually large ventral valve that is referred to this species with some doubt has 

 a length of 9 mm. 



The only traces of the interior of the shell that have been observed are portions of the 

 cardinal area and pedicle furrow of the ventral valve (PL XL VIII, fig. 2j) and the cardinal 

 area of a dorsal valve (PI. XL VIII, figs. 2f and 2 1). 



Observations. — This species is very abundant in the upper beds of the "Tonto" sandstone. 

 It is associated with Oiolus (Westonia) euglypTius (Walcott), and it is often difficult when the 

 two are in the form of imperfect casts, to distinguish between the larger specimens of the two 

 species. They are readily distmguished, when the shells are well preserved, by the difference 

 in surface markings and the more acuminate ventral valves of LinguIeUa lineolata. The latter 

 character, however, is not always of service, especially in the larger shells. In form the ventral 

 valve of this species may be compared with L. acutangula (Roemer) (PL XVII). 



Formation and locality.— Middle Cambrian: (9o) Siliceous limestone about 15 feet (4.5 m.) above the Coronado 

 quartzite [Lindgren, 1905, p. 3], 0.5 mile (0.8 km.) southwest of Milk ranch, on the first spur north of the one which, 

 the main road follows; and (358) argillaceous shale, 50 feet (15.2 m.) above the Coronado quartzite [Lindgren, 1905, 

 p. 3], west side of Chase Creek Canyon, 1.5 miles (2.4 km.) east-northeast of Morenci; both in the Clifton quadrangle 

 (U. S. Geol. Survey), Graham County, Arizona. 



(73) Sandstones of the Tonto group, in Kwagunt Valley; (73a) sandstones of Tonto gToup, in Chuar Valley; (74) 

 sandstone about 300 feet (91.4 m.) above the base of the Tonto group, at the head of Nunkoweap Valley; (74d) sandstone 

 beds in ' ' Tonto " shale, just above massive sandstones near the mouth of Bass Canyon, on the south side of the Grand 

 Canyon, southeast of Powell Plateau; and (17c) sandy limestone 235 feet (71.6 m.) above the "Tonto" sandstone, 

 Grand View trail, north of Last Chance copper mine, south side of the Grand Canyon; all in the Grand Canyon of the 

 Colorado, Arizona. 



LiNGXJLELLA LINNARSSONI (Walcott) . 



Plate XXX, figures 14, 14a. 



Obolus (LinguleUa) Imnarssoni Walcott, 1901, Proc. U. S. Nat. Mus., vol. 23, p. 688. (Described as below as a new 

 species.) 



Ventral valve elongate oval, subacuminate. Surface marked by fine, undulating, depressed, 

 radiating, ridgelike lines; closely undulating, concentric striae; and very fine papillse that 

 appear to terminate in fine sharp points; the papillae are situated on the narrow, irregular, 

 elevated spaces between the striae. Shell relatively thin and formed of several lamellae, more 

 or less oblique to the outer surface. 



Observations. — The species is based on a fine specimen of a ventral valve associated with 

 BiUingsella lindstromi (Linnarsson) in the Paradoxides series of Lovened. It is broader than 

 LinguleUa ferruginea Salter and the surface ornamentation is quite different. The latter is 

 more like that of the associated AcrotJiele coriacea Linnarsson. In outline it approaches more 

 nearly to L. lepis (Salter). 



The specific name was given in honor of Mr. J. G. Linnarsson. 

 Formation and locality. — Middle Cambrian : (320n) Limestones of the Paradoxides forchhammeri zone at Lovened, 

 Djupadal, 19 miles (30.6 km.) south-southeast of Skara, Province of Skaraborg, Sweden. 



