NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 277 



ORTHIS (ORUSIA) LENTICULARIS LYNCIOIDES Matthew. 



Orthk lenticidaris lyncioides Matthew, Trans. Koy. Soc. Caiuula, IX, 1891, p. 49, 

 pi. XII, iigs. lOa-c. 

 Original description. — Distinguished by its sharp, strong, radiating ribs, large umbo, 

 and high cardinal area. In some of the larger examples the radaiting ribs become 

 subordinate to and are replaced on the newer part of the shell by concentric strife, 

 reversing the usual position of the ribbed and the smooth parts of the shells of this 

 species. Size of tlie valves, about 6 by 8 mm. 



Formation and locality. — Same as O. lenticularis in New Brunswick. 



ORTHIS (ORUSIA ?) EUREKENSIS Walcott. 



Orthis eureJcensis Walcott, Mong. U. S. Geol. Survey, VIII, 1884, p. 22, pi. ix, 



fig. 8. 

 Protorthis? eurekensis Hall and Clarke, Pal. N. Y., VIII, 1892, Pt. 1, p. 232. 

 Orthis f eurekensis Schuchert, Bull. U. S. Geol. Sur. No. 87, 1897, p. 286. 



General form rounded .subquadrate; strongly convex. Surface 

 marked by line, rounded, radiating ribs, live in the distance of a mil- 

 limeter. Length of ventral valve, -t mm., width, 4.5; 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 I'eceive 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 

 thick 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 ante- 

 rior 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. 



OhservatioTis. — This strongly-marked shell suggests some of the more 

 convex shells of O. {O.) lenticularis; it differs in its fine ribs; deep 

 median sinus of dorsal valve and sharp fold of 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 some- 

 times indicated in O. {().) lenticularis. 



Form.atu/n and locality. — ^Middle Cambrian. Prospect Mountain 

 limestone just beneath Secret Canyon shale. East slope of Prospect 

 Mountain, Eureka District, Nevada. 



FINKELNBTIRGIA, new subgenus of, ORTHIS. 



This subgenus is based upon two species that difl'er from Plectorthis 

 in having thick shells and strongly marked vascular trunks in the 

 ventral valve. The type of the subgenus, F. finkelnhurgi, with its 

 acuminate and sometimes alate cardinal extremities and subequally 

 convex valves, is a very strongly marked type; the second species, 



