OBOLIDJE. 429 



The vascular markings include the main vascular sinuses in the ventral valve and traces 

 of the visceral area in the dorsal valve. 



This pretty little shell is much like a small species of Ololus in its exterior appearance; 

 the interior connects it with Obolus (Fordinia) perfectus Walcott and 0. (F.) gilberti Walcott. 

 I know of no species closely related to it. 



Formation and locality. — Upper Cambrian: (7x and If) a Limestone of the Emigrant formation [Tunur, 1902, 

 p. 285], about S.'i miles {4 hn.) southeast of Emigrant Pass; and (7z) limestone of the Emigrant formation [Turner, 

 1902, p. 265], about 3 miles (4.8 km.) southeast of Emigrant Pass; both in the Silver Peak quadrangle (U. S. Geol. 

 Survey), Esmeralda County, Nevada. 



Specimens that are somewhat doubtfully referred to Oholus (Fordinia) hellulus occur at the 

 following locality: 



(54f) 150 feet (45.7 m.) above the Middle Cambrian and 1,075 feet (327.7 m.) below the top of the Upper Cambrian 

 in the light-gray sandstone forming No. 4 of the St. Charles formation [Walcott, 1908f, p. 193], in Blacksmith Fork 

 Canyon, about 10 miles (16.1 km.) east of Hyrum, Cache County, Utah. 



Obolus (Fordinia) gilberti Walcott. 



^ Plate LI, figm-e3 5, 5a-d. 



Obolus (Fordinia) gilberti Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 65, PI. VII, figs. 15 and 15a. 

 (Discussed as below as a new species. Figs. 15 and 15a are copied in this mbnograph, PL LI, figs. 5 and 5b, 

 respectively.) 



This shell was first thought to belong with Dicellomus politus (Hall) . It differs from that 

 species in the character of the interior of the dorsal valve and in the narrowing of the umbo 

 as it merges into the apex. The nearest related species is Oholus (Fordinia) hellulus (Walcott). 

 It differs from the latter in being more convex and in the narrowing of the umbo toward the apex. 



The average size of the ventral valve is from 4 mm. to 5 mm. in length by 3 mm. to 4 

 mm. in width. The dorsal valve is a little shorter than the ventral. 



The generic reference is based on the interior of the dorsal vah^e, which is similar to that 

 of 0. (F.) hellulus (Walcott). Associated with this species are specimens so distinct that they 

 are doubtfully referred to the same species. 



The specific name is given in honor of Mr. G. K. Gilbert, geologist, of the United States 

 Geological Survey. 



Formation and locality. — Middle Cambrian: (lln) About 3,000 feet (914.4 m.) above the Lower Cambrian 

 and 1,400 feet (426.7 m.) below the Upper Cambrian, in the upper part of the limestone forming la of the Marjum lime- 

 stone [Walcott, 1908f, p. 179], in the long cliff 2 miles (3.2 km.) southeast of Marjum Pass [Walcott, 1908f, Pis. XIII 

 and XV], House Range, Millard County, Utah. 



. Obolus (Fordinia) perfectus Walcott. 

 Plate LXIII, figures 10, lOa-d. 



Obolus (Fordinia) perfectus Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, pp. 65-66, PL VII, fig. 16. (Described 

 and discussed as below as a new species. Fig. 16 is copied in this monograph, PL LXIII, fig. 10a.) 



General form elongate oval, biconvex; beaks marginal. Surface marked by concentric 

 lines and striae of growth that gather irregularly in small ridges on the anterior two-thirds of 

 adult shells; very fuie, obscure, radiating lines are preserved on some specimens of the outer 

 surface. A shallow, narrow, median sinus occurs on each valve, on which the striae arch slightly 

 backward. Substance of shell apparently calcareocorneous. The shell is strong and built up 

 of numerous layers or lamellEe that, except toward the beaks, are oblique to the outer layer. 



Ventral valve broad ovate, with a rather blunt subacuminate beak; very young shells are 

 broad oval in outline. Area short, and on the plane of the edges of the valve; it is divided 

 midway by a narrow pedicle furrow that interrupts the transverse striae of growth. 



Dorsal valve a little shorter and more rounded at the beak; area short and marked by 

 transverse striae of growth. Both valves moderately convex. 



o7x is the type locality. ' 



