CAMBRIAN FOSSILS. 445 



OBOLUS Eichwald. 



Subgenus LINGULELLA Salter. 



Obolus (Lingulella) desideratus Walcott. 



PI. LX, figs. 2, 2a. 



Obolus (Lingulella) desideratus Walcott, 1898: Proc. U. S. Nat, Mus., Vol. XXI, p. 399. 



Shell small, subovate, with the ventral valve obtusely acuminate, and 

 the dorsal valve broadly ovate. Valves are strongly convexed, with the 

 ventral valve fully as much so as the dorsal. There is a little variation in 

 the outline of the valves, some being slightly more rounded posteriorly 

 than others. 



The surface of the shell is marked by fine, concentric lines of growth 

 and, between them, very fine, slightly irregular striae. A few rather narrow 

 indistinct undulations radiate from the umbo toward the front and lateral 

 margins. When the outer shell is partially exfoliated the outer surface of 

 the inner layer is marked by very fine indistinct radiating striae. There 

 are a few traces of small, scattered pits or punctae on the inner surface of 

 the shell. The shell is thin and is formed of an outer la3^er and one or 

 more inner layers or lamellae. 



The average length of the ventral valve is about 4 mm. ; width, 3 mm. 

 A dorsal valve 3.5 mm. long has a width of 3 mm. 



A cast of the interior of a dorsal valve shows an area of medium 

 length, divided midway by a narrow, clearly defined pedicle groove. The 

 area of the dorsal valve is short. Nothing is known of the interior of the 

 ventral valve, but in a cast of a dorsal valve may be seen traces of 

 the main vascular sinuses and central median ridge, and of the central 

 muscle scars. 



observations. — This species from the Upper Cambrian may be compared 

 with the Middle Cambrian 0. (L.) ferruginens of the Atlantic Basin fauna. 

 Compared with the Rocky Mountain species it is intermediate between 0. 

 (i.) manticulus and 0. (L.) rotundatus. It may also be compared with 0. 

 (X.) granvillensis of eastern New York, upper Olenellus fauna. 



What appears to be an identical species also occurs in the upper beds 

 of the Secret Canyon shale just beneath the Hamburg limestone, 1,200 

 feet lower in the Eureka district Cambrian section. 



