CAMBRIAN RRACHIOPODA WALCOTT 65 



fectiis and the raised area in the dorsal valve is much smaller. In 

 0. (P.) gilbcrti (see below) the thickened areas are much smaller 

 than in the other two species. These three species appear to be 

 forms intermediate between Oboliis and Elkania. 

 Type. — Oboliis {Forciiitia) pcrfcctiis, new species. 



OBOLUS (FORDINIA) GILBERTI, new species 



Plate 7, Figures 15 and 151:1 



This shell was first thought to belong with Diccllomus politiis 

 (Hall) [1861, p. 24]. It diiTers 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 Oboliis 

 (fordinia) bclluliis (Walcott) [1905(7, p. 323]. 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 valve, 

 which is similar to that of O. (F.) bell id us. 



Formation and Locality. — ]\Iiddle Cambrian: About 3,000 feot 

 (914.4 m.) above the top of the Lower Cambrian and 1,400 feet 

 (526.7 m.) below the Upper Cambrian, in gray, more or less thin- 

 bedded limestones of the Alarjum formation [Walcott, 1908a, p. 10], 

 south side of ]\Iarjum Pass, in cliff' southeast of divide. House 

 Range, ]\Iillard County, I'tah. 



OBOLUS (FORDINIA) PERFECTUS, new species 



Plate 7, Figure 16 



General form elongate oval-biconvex : beaks marginal. Surface 

 marked by concentric lines and striae of growth that gather irregu- 

 larly in small ridges on the anterior two-thirds of adult shells ; very 

 fine, obscure, radiating lines are preserved on some specimens of the 

 outer surface. A shallow, narrow^ median sinus occurs on each 

 valve on wdiich the stride arch slightly backward. Substance of shell 

 apparently calcareo-corneous. The shell is strong and built up of 

 numerous layers or lamell?e that, except toward the beaks, are oblique 

 to the outer layer. 



\'entral valve broad ovate, with a rather blunt subacuminate beak ; 

 verv voung shells are broad oval in outline. Area short, and on the 



