OBOLID^. 



455 



Observations. — The character of the surface ornamentation and subquadrate form of the 

 ' dorsal valve lead to a comparison with 0. ( W.) ella (Hall and Wliitfield) (PI. XL VII) of the 

 1 Eocky Mountain fauna. The species differs from the latter in having a thicker, stronger shell, 

 relatively shorter in proportion to its length. The surface is also of the same type as that 

 of 0. (W.) euglijfhus (Walcott) (PI. XL VIII), which occurs at the same horizon in the upper 

 portion of the "Tonto" sandstone but not associated with it. It differs from 0. ( TF.) euglyplius 

 in being much shorter and broader in proportion to its length. 



The species derives its name from its occurrence in the Chuar Valley, Grand Canyon of the 

 Colorado, Arizona. 



Formation and locality. — Middle Cambrian: (74) Sandstone about 300 feet (91.4 m.) above the base of the 

 Tonto group at the head of Nunkoweap Valley; (74b) sandstone about 1,000 feet (305 m.) above the base of the Tonto 

 group, at the head of Nunkoweap Valley; (73) sandstones of the Tonto group in Kwagunt Valley; and (73a) " Tonto " 

 sandstone, in Chuar Valley; all in the Grand Canyon of the Colorado, Arizona. 



IX 



Obolus (Westonia) dartoni (Walcott). 



Text figures 40A-C. 



(Discussed as 



Oholus {Westonia) dartoni Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 67, PI. VII, fig. 14. 

 below as a new species. Fig. 14 is reproduced below, fig. 40A.) 



This species has the general form and convexitj'' of Oholus' ( Westonia) euglyplius (Walcott) 

 (PL XLVIII, figs. 1, la-e). It differs in the dorsal valve being narrower posteriorly. The 

 surface of the two species differs very much, 

 that of 0. ( W.) dartoni being of the 0. ( IF.) 

 ella (HaU and Wliitfield) type (PL XLVII, 

 fig. lo) and not like that of 0. ( W.) euglyplius 

 (PL XLVIII, fig. If). From 0. (TF.) ella 

 this species differs in being more elongate 

 in outline and in having the surface more 

 clearly marked by the crossing of the minute 

 ridges. These ridges are slightly irregular 

 and curve from near the umbo obliquely 

 across the shell toward the lateral and front 

 margins so as to form a pattern much like 

 that shown on the front portion of Plate 

 XLVII, figure lo. Some portions of the 

 surface recall that of the central portions 

 of Plate XLVI, figure Ih. The largest ventral valve has an indicated length of 12 to 15 

 mm.; width, 9 mm. 



The specific name was given for Mr. N. H. Darton, of the LTnited States Geological Survey, 

 who collected the specimens. 



Formation and locality. — Middle Cambrian: (303o) Sandstones just above the granite, west of Garfield Peak, 

 50 milea (80.5 km.) west of Casper, Natrona County, Wyoming. 



Obolus (Westonia) ella (Hall and Whitfield.) 



Plate XXXIII, figures 3, 3a-c; Plate XLVII, figures 1, la-p. 



Lingulepis ella Hall and Whitfield, 1877, U. S. Geol. Expl. 40th Par., vol. 4, p. 232, PL I, fig. 8. (Described and 

 discussed as a new species. The specimen represented by fig. 8 is redrawn in this monograph, PL XLVII, fig. lb.) 



Lingulella ella (Hall and \Vhitfield), Walcott (in part), 1886, Bull. U. S. Geol. Survey No. 30, pp. 97-98, PL VII, 

 fig. 2; PL VIII, figs. 4, 4a, 4d, and 4e (not figs. 4b and 4c, now referred to Lingulella dubia). (Copies original 

 description and describes and discusses species. Fig. 4 is redrawn from the specimen represented in fig. 8 of 

 the preceding reference. The specimens represented by figs. 2 and 4 are redrawn in this monograph, PL 

 XLVII, figs. Im and lb, respectively.) 



Lingulella ella (Hall and Whitfield), Walcott (in part), 1891, Tenth Ann. Rept. U. S. Geol. Survey, p. 607, PL LXVII, 

 figs. 2, 2a-b, and 2e (not figs. 2c and 2d, now referred to Lingulella dubia). (Horizons mentioned. Figs. 2, 2a-b, 

 and 2e are copied from figs. 2, 4a, 4, and 4e, respectively, of the preceding reference.) 



A 



Figure 40. — Oholus {Westonia) dartoni Walcott. A, Ventral valve, worn 



smooth by beach action (U. S. Nat. Mus. Cat. No. 51GS3a). B, Dorsal 



valve, worn smooth (U. S. Nat. Mus. Cat. No. 51GS3b). C, Enlargement ol 



the sm-face, X 10 (U. S. Nat. Mus. Cat. No. 51683c). 



The specimens represented are from Locality 302o, Middle Cambrian 



sandstones near Garfield Peak, Wyoming. Figure 40.\ is copied from 



Walcott [1908d, PI. VII, flg. 14]; it represents the type specimen. 



