CAMBRIAN BRACK lOPODA — WALCOTT 67 



It occurs over 1,000 feet (304.8 m.) higher up in the section of the 

 ^Middle Cambrian Hmestones than O. (F.) gilbcrti. The interior of 

 its ventral valve is somewhat like that of O. {F.) belhdus (Walcott) 

 [1905. p. ^2T,], but it differs from that and all species of Fordinia 

 in having in both valves a cardinal area that has not been merged into 

 the reversed area of the ventral valve as in the other species referred 

 to Fordinia. 



Formation and Locality. — Middle Cambrian: About 3,750 feet 

 (1,143 m.) above the top of the Lower Cambrian and 650 feet 

 (198. 1 m.) below the Upper Cambrian, in the shaly limestones of 

 the Weeks formation [Walcott, 19080, p. 10], north side of Weeks 

 Canyon, 3.5 miles (5.63 km.) south of Marjum Pass, House Range, 

 Millard County, Utah. 



Subgenus WESTONIA Walcott [1901, p. 683] 

 OBOLUS (WESTONIA) DARTONI, new species 



Plate; 7, Figure 14 



This species has the general form and convexity of Oholns {Wes- 

 toiiia) ciiglyphus (Walcott) [18986, p. 402], but it differs in the 

 dorsal valve being narrower posteriorly. The surface of the two 

 species dift'ers very much, that of O. {W.) dartoni being of the O. 

 (JVcstonia) ella (Hall and Whitfield) [1877, P- 232] type and not 

 like that of 0. (W.) euglyphus. From O. (W.) ella this species 

 dift'ers 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. The largest 

 ventral valve has an indicated length of from 12 to 15 mm.; width, 

 9 mm. 



The specific name is given for Mr. N. H. Darton, of the U. S. 

 Geological Survey, who collected the specimens. 



Formation and Locality. — Middle Cambrian : Sandstones just 

 above the granite, west of Garfield Peak, 50 miles (80.47 km.) west 

 of Casper, Natrona County, Wyoming. 



OBOLUS (WESTONIA) ELLA ONAQUIENSIS, new variety 



This variety is represented by a number of more or less imperfect 

 specimens that at first sight might be placed with Obolus {Westonia) 

 ella (Hall and Whitfield) [1877, p. 232], but the character of the 

 surface clearly distinguishes the two forms. In typical forms of O. 

 {JV.) ella the transverse striae are more regular, while in this variety 



