CAMBRIAN BRACHIOPODA. 



opening minute, elongate, and situated on the slope just in front of the beak. Dorsal valve 

 a trifle less convex than the ventral. 



The outer surface of the shell is marked by fine concentric striae and lines of growth, and 

 the inner lamellse and inner surface are marked by fine radiating strise. The shell is formed 

 of several thin layers or lamellse, those of the anterior and lateral portions being more or less 

 oblique to the outer surface layer; the shell is also thickened in the visceral region by irregular 

 additions on the inside. The average length of the ventral valve is 2 to 2.5 mm.; the dorsal is 

 a little shorter. The interior of the ventral valve shows a cardinal scar on each side well out 

 toward the lateral margin, which corresponds to the position of the transmedian and antero- 

 lateral scars in Oiolus apoUinis. The visceral area is small and compressed between the sinuses 

 occupied by the large, maiti vascular canals. The irmer opening of the pedicle tube is oval 

 and situated at the posterior portion of the visceral depression at the margin of the area. The 

 main vascular canals start near the beak and gradually diverge toward the anterolateral 

 margins of the valve. The interiors of the dorsal valve associated with the two ventral valves 

 illustrated are all so imperfect that only a median ridge like that in Acrotreta can be clearly 

 determined; this ridge varies greatly in size and length in different specimens. 



Ohservations. — In a hasty examination of this species [1884b, p. 13] I confused the dorsal 

 valve with that of Acrotreta and considered the ventral valve as the type, placing it under 

 Lingulepis, stating that it had nearly the same vertical range and geographic distribution as 

 Oholus msera. As now known it is confined to the following localities, one of the two being the 

 locality discovered by the geologists of the Fortieth Parallel Survey. 



The generic reference is somewhat doubtful on account of our limited knowledge of the 

 interior of the dorsal valve and the position of the area of the ventral valve. 



Formation and locality. — Ordovician: (313k) Limestone in the White Pine district, Nye County, Nevada. 

 TTpper Gambiian: (3131i) Sandy shale on Hamburg Ridge, Eureka district [Hague, 1892, Atlas], Eureka County, 

 Nevada. 



LiNNARSSONELLA MODESTA Walcott. 



Plate LXXVIII, figures 8, 8a-t. 



Linnarssonella modesta Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 90-91, PI. IX, figs. 8 and 8a. (De- 

 scribed and discussed as below as a new species. Figs. 8 and 8a are copied in this monograph, PL LXXVIII, 

 figs. 8 and 8e, respectively.) 



Shell minute; general form subcircular. Ventral valve moderately convex; beak small 

 and shghtly incur s^ed over a very low false area; pedicle opening minute exteriorly, with a 

 sHghtly elevated, sharp ridge on each side a httle in advance of the point of the beak; seen 

 from the back, the minute aperture points backward from the bottom of a pit between the 

 two short, sharp ridges; in a specimen from a layer of limestone a short distance above in 

 the section, the aperture is at the point of a small nipple-Hke projection, apparently formed 

 by the union of the ends of the ridges already described on each side of the aperture. Dorsal 

 valve slightly convex, with a minute beak at the posterior margin. 



Surface dull when the outer layer of ths shell is preserved; it is marked by a few very 

 fine, concentric lines of growth. The inner surface is marked by concentric lines and a few 

 fine radiating lines. The shell is formed of several layers or lamellae and is relatively thin. 



The average diameter of the adult shell is from 1.75 to 2.25 mm.; the dorsal valve shghtly 

 shorter than the ventral. 



The cast of the interior of the ventral valve shows a slight, longitudinal, median elevation 

 and two short, not very strongly marked main vascular sinuses; several specimens indicate 

 a slight cavity just beneath the aperture, which probably represents the inner side o£ the 

 pedicle tube. The interior of a dorsal valve shows main vascular sinuses quite close to the 

 outer margin; also two small, rounded median depressions a short distance in front of the 

 posterior margin. 



Ohservations. — This shell occurs in great abundance on the surface of shaly, gray lime- 

 stone, about midway of the strata referred to the Upper Cambrian in the House Range 



