NO. 1395. CAMBRIAN BRACHIOFODA—WALCOTT. 237 



BILLINGSELLA HICKSI (Saitcr^^Davidson.^ 



OrlMs liicksi (Salter MS.) Davidson, Geol. Mag., V, 1868, p. ol4, pi. xvi, figs. 



17-19. 

 (Mills hicksi Davidson, Brit. Foss. Bracli., Ill, 1871, p. 230, ])1. xxxiii, figs. 



13-16. 

 Ortlilx liicksi Kayskk, Cambrische Brach. v. Lian Tung, China, Kiclitliofen, IV, 



1893, p. 35. 



Origltial description. — Shell small, transversely oval; hinge-line shorter than the 

 greatest breadth of the shell; cardinal angles rounded. Dorsal valve semicircular, 

 moderately convex, slightly longitudinally depressed along the middle. Ventral 

 valve convex, deeper than the opposite one. Area triangular, moderately wide; sur- 

 face of valves ornamented by about ten principal, narrow, radiating ribs, with wide 

 interspaces between each pair, in the middle of which is situated a shorter rib. 



Length about 4, width 5 lines. 



Obs. — Orthis hicksi is a scarce fossil, and very rarely found, even in a passably com- 

 plete condition. It was discovered by Mr. Hicks in the middle (sandstone) beds of 

 the Menevian group at Ninewells and Porth-y-rhaw, near St. Davids, and is the oldest 

 species of the genus on record. It has not been hitherto found in North Wales, nor, 

 to my knowledge, in any other locality. 



This shell is most nearly related to £. exjporrecta. It differs in its 

 more rounded form, fewer radiating ribs and muscle area of ventral 

 valve. 



Formation and locality. — Middle Cambrian. Menevian sandstone, 

 as stated above. 



BILLINGSELLA HIGHLANDENSIS Walcott. 



Orthis ' highlandensis Walcott, Bull. U. S. Geol. Sur. No. 30, 1886, p. 119, pi. 



viii, figs. 3, 3a, 3b. 

 Orthis highlandensis Walcott, Tenth Ann. Rept. U. S. Geol. Sur., 1891, p. 612, 

 pi. Lxxii, figs. 5, 5a, 5b. 



Shell transversely oval or subrotund. Front broadly rounded and 

 nearly straight in the dorsal valve. On the ventral valve the cardinal 

 line slopes toward the beak at an angle of about 20^, while in the dor- 

 sal valve it is less than 10°. The greatest width is about the middle of 

 the shell. The ventral valve is moderately convex, being most ele- 

 vated toward the beak, which is slightly arched over to the cardinal 

 margin. 



A dorsal valve associated in the same hand specimen of limestone is 

 more convex than the ventral valve. The cardinal line is shorter at 

 the greatest width of the shell. The median fold is but slightly raised 

 above the general surface, although the front margin has a low, broad 

 arch for the reception of the fold of the ventral valve. 



There is a slight flattening in the central portion of the ventral valve 

 that in some specimens forms a low, broad, mesial sinus toward the 

 front margin. 



The surface is marked by concentric lines of growth; otherwise it 

 appears to be smooth. Interior casts of the valves show fine, radiat- 

 Proc. N. M. vol. xxviii— 04 16 



