NO. 1395. 



CAMBRIAN BRA CHTOPODA— WALCOTT. 



259 



The species referred to the .subgenus are as follows: 



Cambrian. 



M. 



f atava Matthew 



vhrislianix Kjerulf 



dauniDs, new species 



desmopleitra Meek 



desmopleura nympha, new variety 



diablo, new species 



doris, new species 



fiastingsensis, new species 



iddingsi, new species 



indianola, new species 



johannensis Matthew 



kayseri, new species 



Urinarssoni, new species 



newtonensis Weller 



pagoda, new species 



papias, new species 



remnicha Winchell 



remnicha sulcata, new variety 



remnicha texana, new variety 



remnicha winfieldensis, new variety 



retroflexa Matthew 



saltensis Kayser 



tidlbergi, new species 1 i 



urichitaensis, new species ! j x 



ivichitaensis laeviusculus, new variety - x 



wimani, new species ! 



2 species undetermined 



PLECTORTHIS? ATAVA Matthew. 



Slrnphomena atava Matthew, Trans. Roy. Soc. Canada, X, 1898, p. 102, pi. 



VII, fig. 8a-f. 

 Rafinesquina ? akiva Schuchert, Bull. U. S. Geol. Sur. No. 87, 1897, p. 338. 



Description by Doctor Matthew. — Narrowly 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 inconspicuous 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 ridges 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 half way to the front of the valve. There is a 

 low sharp ridge at the umbo, between the branches of the posterior cardinal nmscle, 

 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 



