774 CAMBKIAN BEACHIOPODA. 



Surface marked by rounded radiating ribs and interspaces, 6 ribs in a space of 3 mm. 

 near the front margin; a few of the ribs bifurcate, but most of them extend from the umbo 

 to the front margin; the ribs are crossed by fine concentric striae and strong lines of growth. 



Observations. — This shell is characterized by its regular convexity and the uniformity of 

 the rounded, radiating ribs. 



Formation and locality. — Middle Cambrian: (C28) Upper part of the thin-bedded gray limestone at the base 

 of the Changhia formation [Blackwelder, 1907a, p. 32 (first list of fossils), and fig. 6 (bed 20), p. 25] 1 mile (1.6 km.) 

 east-southeast of Changhia, Shantung, China. 



EOORTHIS ATAVA (MattllCw). 



W 



Plate XCV, figures 7, 7a-b. 



Strophomena atava Matthew, 1893, Trans. Roy. Soc. Canada for 1892, 1st ser., vol. 10, sec. 4, No. 7, pp. 102-103, PI. 

 VII, fig. 8a-f. (Described and discussed as a new species; see below for copy. The specimens represented 

 by figs. 8c, 8a, and 8b are redrawn in this monograph, PL XCV, figs. 7, 7a-b, respectively.) 

 Rafinesquina? atava (Matthew), Sohucheet, 1897, Bull. U. S. Geol. Survey No. 87,.,p. 338. (Merely changes generic 



reference.) 

 Orihis {Plectorthis?) atava (Matthew), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 259-260. (Original descrip- 

 tion copied and species discussed as below.) 

 Strophomena? atava Matthew, Mobeeg and Segbrberg, 1906, Medd. bka Lunds Geol. Faltklubb, Ser. B, No. 2 

 (Aftryck ur K. Fysiografiska Sallskapets Handl., N. F., Bd. 17), PI. II, figs. 7, 7a-b. (No text reference. Figs. 

 7, 7a-b, in common with the remaining figures on PL II of Moberg and Segerberg's paper, were copied from 

 a preliminary photograph of PL XCV of this monograph.) 

 The original description by Matthew follows : 

 Narrow semicircular. Valves slightly arched, rather wider than the hinge line, compressed at the cardinal 

 angles; sides of the valves nearly straight behind, regularly rounded in front. 



Ventral valve moderately convex, highest and somewhat angulated at the middle of its length. Umbo incon- 

 spicuous and appressed to the very narrow area. 



Dorsal valve somewhat concave, especially toward the umbo, which is depressed to the general level of the 

 valve. 



The interior of the ventral valve has characters intermediate between those of Orthis and Strophomena. The 

 rido-es inclosing the cardinal muscles are shorter and closer at their extremities than in Strophomena, and the scar of 

 the adductor muscle is broader and shorter, but extends halfway to the front of the valve. There is a low, sharp 

 ridge at the umbo between the branches of the posterior cardinal muscle, and the ventral cavity is faintly outlined 

 in front. Another cast of the ventral valve of a larger individual, perhaps of this species, shows the impressions of 

 the muscles more distinctly; in this the impression of the adductor muscle is much nearer to the hinge, being about 

 two-thirds from the front of the valve. There is a low callus in front of the imprint of the adductor muscle. 



A partly decorticated example of the dorsal valve exhibits a small bifid scar at the hinge line, due to the adductor 

 muscle. There is no median ridge, such as is found in Orthis, the median sinus being almost obsolete. In front of the 

 impression of the adductor muscle is a small, low callus. 



Sculpture: This consists of sharply raised, rounded, radiating ridges, slightly crenulated by obscure, transverse 

 lines. These ribs are of varying size, every third or fifth rib being larger than the others. These large ribs extend 

 outward from the umbo, and the smaller ones are intercalated between them, or spring from them. On the dorsal 

 valve there are two main ribs in the sinus of the valve and three others extending to the lateral margins; smaller ribs 

 are intercalated between these. On each side of the group of ribs on the middle third of the shell are four fainter, 

 but more strongly arched groups of ridges which throw off minor ridges at the back, directed toward the lateral mar- 

 gins. On the ventral valve the small ribs are alrdost all intercalated and do not spring from the back of the primary 

 ribs, as in those of the dorsal valve. 



Size: Length, 12 mm.; width, 14 mm. Height of area of dorsal valve, 0.5 mm.; of ventral valve, 1 mm. A 

 valve supposed to be of this species is 15 mm. long and 19 mm. wide. 



Ohservations. — With the available data this species appears to be quite as near to Eoorthis 

 as any described genus. It does not appear to belong with the typical forms of Strophomena 

 or Rafinesquina. The surface ribs, increasing by interpolation, and possibly bifurcation, and the 

 form of the pseudospondylium in the ventral valve, strongly suggest a relationship with Eoorthis. 

 It will probably require better preserved material to determine satisfactorily the correct 

 generic reference for the species. 



Formation and locality. — Upper Cambrian: (308a [Matthew, 1893b, p. 103]) Dark shales of division C3a of 

 Matthew's section of the St. John formation, Navy Island, St. John Harbor, St. John County, New Brunswick, Canada. 



