DESCRIPTION OF GENERA AND SPECIES. 83 



This species is distinguished from all other species by the strong, incurved 

 umbo, and rounded ribs with narrow interspaces. 



Formation and Locality. Middle Cambrian: (C75) Limestone near the base 

 of the Ki-chou formation [Willis and Blackwelder, 1907, p. 143], 4.5 miles (7.2 km.) 

 south of Wu-t'ai-hien, Shan-si, China. 



Collected by Eliot Blackwelder. 



Eoorthis linnarssoni (Kayser). 



Plate 4, Figures 1 1, i ia. 

 Orthis linnarssoni KAYSER, 1883, China, Richthofen, vol. iv, p. 34, plate in, fig. i. (Described and 



discussed in German as a new species ; see below for translation . Fig. i is reproduced by Walcott , 



191 1, plate xcvn, fig. 50.) 

 Orthis (Plectorlhis) linnarssoni (Kayser), WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxvin, p. 266. 



(Original description copied and species discussed as below.) 

 Eoorthis linnarssoni (Kayser), WALCOTT, 1912, Monogr. U.S. Geol. Survey, vol. Li.pp. 782-783, plate 



xcvn, figs. 5, 5<7. (Copy of the preceding reference.) 



The original description by Kayser follows: 



"Shell semielliptical in outline, broader than long, with hinge edge straight, 

 corresponding to the greatest breadth of the shell. Large (ventral) valve moder- 

 ately convex. Small (dorsal) valve slightly arched, with a sinus which develops 

 at the umbo, and grows quite broad and deep toward the edge. Umbo of ventral 

 valve small, area very low. Surface of shell covered with rather sharp ribs, exceed- 

 ingly variable in strength, separated by narrow furrows. By reason of repeated 

 splitting, which begins close to the umbo, the ribs appear more like bundles of ribs. 

 On the matrix these bundles appear as broad, obtuse-angled folds, which, owing to 

 repeated marginal splitting in even a higher degree than on the shell itself, appear 

 as bundles of ribs of very unlike strength. When the surface is well preserved, a 

 delicate concentric growth striation is perceptible." 



Doctor Kayser compares this shell with Billingsella liicksi (Salter) [Walcott, 

 igi2b, plate xcvn, figs. 7, "ja-b] and Billingsella exporrccta (Linnarsson) [idem, plate 

 LXXXVIII, figs, i, i a-/], on account of the surface characters. It does not appear 

 to be very closely related to either species, but it is in many respects allied to 

 Eoorthis desmoplcura (Meek) [idem, plate xcvi, figs, i, ici-r] and E. wichitaensis 

 (Walcott) [idem, plate xciv, figs, i, ia-0, in]. Eoorthis linnarssoni differs from both 

 in being more transverse. This, species is represented by a dorsal valve, illustrated 

 by figure 1 1 , from near Yen-chuang. 



Formation and Locality. Upper Cambrian: (064) Upper limestone member 

 of the Kiu-lung group [Blackwelder, 19070, pp. 37 and 42 (first list of fossils), and 

 fig. 10 (bed 20), p. 38], 2.7 miles (4.3 km.) southwest of Yen-chuang, Sin-t'ai district, 

 Shan-tung, China. 



Another dorsal valve that is very closely related to, if not identical with this 

 species, occurs in stream gravels at Locality ('67, Upper Cambrian: stream gravels 

 (these are taken from the wash from the mountains south of the city) used in making 

 the railroad grade one-third mile (0.5 km.) west of the west city gate at Tsi-nan, 

 Shan-tung, China. 



Collected by Eliot Blackwelder. 



Eoorthis pagoda (Walcott). 



Plate 4, Figures 12, I2a-d. 

 Orthis (Plectorthis} pagoda WALCOTT, 1905, Proc. U.S.Nat. Mus., vol. xxvin, pp. 267-268. (Described 



and discussed as on page 84 as a new species.) 



Eoorthis pagoda (WALCOTT), 1912, Monogr. U. S. Geol. Survey, vol. LI, pp. 784-785, plate xcvn, figs. 

 12, i2a-d. (Copy of the preceding reference.) 



