406 CAMBRIAN BEACHIOPODA. 



The specific name was given in honor of Alex. Murray, who discovered the specimen. 



Formation and locality. — lower Ordovician: (314g [Billings, 1865b, p. 362]) A loose piece of gray argillaceous 

 limestone at Maiden Arm, Hare Bay, on the east side of the northern point of Newfoundland. 



Obolus namouna Walcott. 



'■J 



Plate VIII, figures 2, 2a. 



Obolus namouna Walcott, 1898, Proc. U. S. Nat. Mus., vol. 21, p. 390. (Discussed as below as a new species.) 



This form is closely related to 0. matinalis (Hall). It differs mainly in the internal char- 

 acter of the dorsal valve (PI. VIII, fig. 2a). The area, in addition to the narrow area of 

 0. matinalis, extends its lines of growth nearly one-fifth the length of the shell. The visceral 

 area is shorter also than in 0. matinalis, the central and anterior lateral scars being closer 

 together. Owing to the somewhat imperfect character of the ventral valve, no special points 

 of difference with the ventral valve of 0. matinalis can be determined. It is associated on 

 the same slabs of sandstone with 0. rJiea Walcott. 



Formation and locality. — Upper Cambrian : (98) "St. Croix sandstone" near Eau Claire, Eau Claire County, 



Middle Cambrian: (84) "St. Croix sandstone" at Dresbach, opposite the mouth of Black River, Winona Coimty, 

 Minnesota. 



Obolus nundina Walcott. 



Plate XI, figures 4, 4a. 



Obolus nundina Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 326. (Characterized and discussed as below as a 

 new species.) 



This little shell differs from Oholus anceps Walcott, to which it appears to be most nearly 

 related, by its more nearly circular dorsal valve and its strong, concentrically ridged surface. 

 Its surface is formed by elevated, rounded, concentric ridges that are somewhat irregular. 

 Very fine concentric striae occur both on the ridges and on the interspaces between. 



Formation and locality. — Upper Cambrian; (61) Limestone in the Dunderberg shale [Walcott, 1908f, p. 184] 

 a little south of the Hamburg mine. Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



(14b) Limestone on Cold Creek at north end of gorge opposite the north end of Sponge Mountain, 2 miles (3.2 km.) 

 south of the San Saba County line, in Llano County, Texas. 



Middle Cambrian: (58) Shaly limestones in the upper beds of Secret Canyon shale, east side of New York and 

 Secret canyons. Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



Obolus obscurus Walcott. 



V 



Plate XI, figures 9, 9a-d. 



Obolus obscurus Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 326. (Described and discussed essentially as in 

 the first two paragraphs below as a new species.) 



A large dorsal valve of this species has a length of 9 mm., width 7 mm. The shell is 

 strong and marked on its inner layers by radiating and concentric strise ; the outer surface has 

 numerous fine, elevated, slightly irregular, concentric striae that, with a strong lens, give it a 

 roughened appearance. 



An interior of a dorsal valve shows a well-developed area, the greater portion of which 

 is taken by the broad pedicle groove; a deep umbonal cavity with a narrow median septum 

 and obscure main vascular sinuses on the outer margins of the cavity. Outside the sinuses, 

 on the posterolateral slope, relatively large muscle scars are indicated. 



Since the publication of the original description (the greater part of which is copied in 

 the two preceding paragraphs), better specimens referred to tliis species were received from 

 the Middle Cambrian of Shansi. These illustrate the form and convexity of the ventral valve 

 and show it to be similar to that of Obolus chinensis (Walcott). The concentric ridges are 

 more nearly perfect than those on the fragments associated with the specimens from Shantung. 

 The concentric striae or ridges are elevated, roimded, with sharp depressions between them; 



