BILLINGSELLID^. 751 



187], in the pass about 7 miles {11.2 hm.) east of Resting {Freshwater) Springs; and (14p) sandstones near Resting (Fresh- 

 water) Springs; bothi in the southwest corner of T. 21 N., R. 8 E., on Amargosa River, in the southeastern part of Inyo 

 County, California. 



BiLLiNGSELLA coLORADOENSis (Shumard). 



Text-figures 6, page 299, 66; Plate LXXXV, figm-es 1, la-z. 



Orthis coloradoensis Shumard, 1860, Trans. Acad. Sci. St. Louis, vol. 1, p. 627. (Described as a new species.) 



Orthis pepina Hall, 1863, Sixteenth Ann. Rept. New York State Cab. Nat. Hist., pp. 134-135, PI. VI, figs. 23-27. 

 (Described and discussed as a new species.) 



Orthis pepina Hall, 1867, Trans. Albany Inst., vol. 5, p. 113, PL I, figs. 23-27. (Copy of preceding reference.) 



Orthis pepina Hall, Whitfield, 1882, Geology of Wisconsin, vol. 4, pt. 3, pp. 170-171, PL I, figs.. 4 and 5. (Described 

 and discussed.) 



Orthis? {Orthisina?) pepina Hall, 1883, Second Ann. Rept. State Geologist New York for 1882, PL XXXVII, figs. 

 16-19. (Figs. 16-19 are copied from Hall, 1863, PL VI, figs. 23, 25, 24, and 26, respectively.) 



Billingsella pepina (Hall), Hall and Clarke, 1892, Eleventh Ann. Rept. State Geologist New York for 1891, PL VIII, 

 figs. 1 and 2. (No text reference.) 



Billingsella pepina (Hall), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 230, PL VII, 

 figs. 16-19; PL VIIA, figs. 7-9. (Mentioned in description of genus. PL VII, figs. 16-19, are copied from 

 Hall, 1863, PL VI, figs. 23, 25, 24, and 26, respectively. Figs. 8 and 9 are copied from figs. 1 and 2, respec- 

 tively, of the preceding reference.) 



Orthis {Billingsella) pepina (Hall), Sardeson, 1896, Bull. Minnesota Acad. Nat. Sci., vol. 4, No. 1, pt. 1, p. 96. (New 

 localities mentioned.) 



Billingsella coloradoensis (Shumard), Schuchert, 1897, Bull. U. S. Geol. Survey No. 87, p. 158. (Merely changes 

 ■ generic reference.) 



Billingsella coloradoensis (Shumard), Walcott, 1899, Mon. U. S. Geol. Survey, vol. 32, pt. 2, pp. 450-451, PL LXI, 

 figs. 1, la-d. (Discussed and characterized. Figs. 1, la-d are copied in this monograph, PL LXXXV, figs. 

 la, Ig, Iw, lo, and It, respectively, figs. Iw and It being slightly altered.) 



Billingsella coloradoensis (Shumard), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 231-234. (Described and 

 discussed essentially as below.) 



Billingsella coloradoensis (Shumard), Grabau and Shimee, 1907, North American Index Fossils, vol. 1, p. 210. (Char- 

 acterized.) 



Shell usually transverse but in many examples the ventral valve is longer than wide. 

 The general outline is irregularly subquadrate to subsemicircular. On the ventral valve the 

 hinge line slopes toward the beak at a low angle while in the dorsal valve it is nearly straight. 

 In some individuals the greatest width is at the hinge line. 

 In others it is at about the middle. There is considerable 

 variation in the relative proportions of length and breadth. 



The ventral valve is slightly more convex than the 

 dorsal. The degree of convexitj^ of the two valves varies 

 considerably m specimens irom dinerent localities, borne or posterior view of a ventral vaive showing 

 the ventral valves from beds at Franconia, Minnesota, and pedicle opening in the deitidium; from Locai- 



,1 1 n i! m 1 TUT- ■ J. 1 ity 70, Upper Cambrian, on Morgans Creek, 



the young shells from Trempealeau, Wisconsin, are strongly T^^g^ (u. s. Nat. mus. Cat. No. 34777a). 

 convex. 



A low, broad, median sinus occurs on nearly all specimens of the dorsal valve, being straight 

 in the young shells, and occasionally a shallow sinus is clearly defined on the ventral valve. 

 It is quite rare to find a well-marked median fold on the ventral valve. 



The surface is marked by sharply rounded, radiating costse crossed by fine concentric 

 lines of growth and very fine, slightly undulating, concentric stride . The size and sharpness of 

 the radiating ribs vary greatly in specimens from the same bed at the typical locality in Texas. 

 Shells occur with strong rounded costse, and no traces of fine elevated lines between, whereas 

 others show from one to four or more lines that start at varying distances from the beak. The 

 imbricating lines of growth give a concentrically ridged aspect to some shells, though others 

 are nearly smooth from beak to frontal margin. The increase in the number of ribs is by 

 interpolation, not by bifurcation. 



There is considerable variation in the size of the shell. The largest specimens from Texas 

 have a transverse diameter of 19 mm., with a length of 14 mm. for the dorsal valve, and about 

 18 mm. for the ventral valve. Specimens of the ventral valve from the Yellowstone National 

 Park have a length of 14 mm. with a width of 15 mm. 



