BILLINGSELLIDiE. 



765 



Genus OBTJSIA Walcott.a 



Orthis {Orusia) Walcott, 1905, Prpc. U. S. Nat. Mus., vol. 28, p. 273. (Characterized and discussed essentially as 



below as a new subgenus.) 

 Orusia Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 148. (Classification of genus.) 



Orthis lenticularis of authors is not an Orthis as restricted by Hall and Clarke [1892c, 

 pp. 192-194]. It appears to belong more nearly to the group of species assembled under 

 Eoorthis. It departs from Eoorthis in its very thin shell, subequally convex valves, small 

 umbonal cavity, and Avide variation in form and surface characters. 



Type. — Anomites lenticularis Wahlenberg [1821, p. 66]. 



Orusia? eurekensis (Walcott). 

 Plate CI, figures 7, 7a. 



Orthis eurekensis Walcott, 1884, Mon. U. S. Geol. Survey, vol. 8, pp. 22-23, PI. IX, figs. 8 and 8a. (Described as 



a new species. The specimens represented by figs. 8 and 8a are redrawn in this monograph, PI. CI, figs. 7a and 



7, respectively.) 

 Protorthis? eurekensis (Walcott), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 232. 



(Merely changes generic reference.) 

 Orthisf eurekensis Walcott, Schuchekt, 1897, Bull. U. S. Geol. Survey No. 87, p. 286. (Merely accepts the original 



generic reference.) 

 Orthis (Onisia?) eurekensis Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 277. (Described and discussed as below. 



General form rounded subquadrate; strongly convex. Surface marked by fine, rounded, 

 radiating ribs, five in the distance of 1 mm. Length of ventral valve, 4 mm.; width, 

 4.5 mm.; dorsal valve a trifle shorter. 



Ventral valve with a high median fold over the umbo, that widens and flattens toward 

 the front margin, which is arched to receive the projecting median depression of the dorsal 

 valve. The umbo curves over to the apex, which overhangs the hinge line. Area low and 

 strongly inclined backward. The interior cast shows that there was a tliick pseudospondylium, 

 but no details of it are preserved. 



Dorsal valve with a deep, gently curved median sinus or depression that extends from 

 the umbo to the front margin. In the cast strong anterior adductor muscle scars are shown 

 on the sides of the median depression, and just back of them, in the sinus, what may be the 

 smaller posterior adductor scars. 



Observations. — This strongly marked shell suggests some of the more convex shells of 

 Orusia lenticularis (Wahlenberg); it differs in its fine ribs, deep median sinus in the dorsal valve, 

 and the sharp fold of the ventral valve; also in the presence of a thickened shelf beneath the 

 umbo of the ventral valve that suggests a spondylium; this character, however, is sometimes 

 indicated m 0. lenticularis (PI. XCVIII, figs. Ic and le). 



The specific name is derived from Eureka district, Nevada. 



Formation and locality. — Middle Cambrian: (55b) Top of the Eldorado limestone [Walcott, 1908f, p. 184] 

 west side of Secret Canyon, Eureka district [Hague, 1892, Atlas], Eureka County, Nevada. 



Orusia lenticularis (Wahlenberg). 

 Plate XCVIII, figures 1, la-p, 2, 2a-k, 3, 3a-b, 6, 6a-c. 



Anomites lenticularis Wahlenberg, 1821, Nova Acta Regies Soc. Sci. Upsaliensis, vol. 8, Petrificata telluris Svecanse, 

 pp. 66-67. (Described in Latin as a new species; see Matthew, 1892, p. 46, for copy of or?ginal, and translation.) 



Atrypa? lenticularis (Wahlenberg), Dalman, 1828, K. Svensk. Vet. -Acad. Handl. for 1827, pp. 132-133. (Described 

 in Latin; see Matthew, 1892, p. 46, for copy of original, and translation.) 



I Prior to the deflnition of the genus Orusia, the species now placed 

 Anomites Wahlenberg [1821, p. 66]. 

 Atrypa? Dalman [1828, p. 132].- 

 Spirifcraf von Buch [1834, p. 48]. 

 ■ Atrypa? Hisinger [1837, p. 76]. 

 Atrypa Kjerulf [1857, p. 92; 1866, p. 3). 

 Orthis Salter [1866, p. 339]. 

 Orthis Davidson [1868, p. 314; 1869, p. 230]. 

 OrtJiis Kayser [1876, p. 9]. 

 Orthis Roemer [1876, PI. II, figs. 4a-c]. 



under that genus were referred to the following genera: 

 Atrypa Kjerulf [1S79, PI. XIII]. 

 Orthis Salter and Etheridge [1881, p. 544]. 

 Orthis Brogger [1882, p. 48]. 

 Orthis Walcott [lS84b, p. 22]. 

 Orthis Roemer [1S85, p. 33]. 

 Orthis Matthew [1892, pp. 46, 48, and 49]. 

 Protnrthis? Hall and Clarke [1892c, p. 232]. 

 Orthis? Schuchert [1897, p. 286]. 

 Orthis Matthew [1903, pp. 213, 216, and 217]. 



