744 CAMBRIAN BRACHIOPODA. 



Only one specimen of a cast of a ventral valve of this shell was found. It suggests by its 

 convexity and surface some of the more convex forms of Protorihis quacoensis 



Formation and locality. — Upper Cambrian: (75) Thin-bedded limestone just below the base of the Ordovician, ' 

 in the Tonto group; near the water's edge at the mouth of Kanab Canyon, Grand Canyon of the Colorado, Arizona. 



LOPEBIA Walcott, subgenus of PROTORTHIS. 



Proiorthis (Loperia) Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, p. 287. (Chai-acterized as below as a new sub- 

 genus.) 



Protorihis {Loperia) Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 147. (Classifica- 

 tion of subgenus.) 



This subgenus is characterized by the form of the valves. The ventral valve is elevated 

 at the umbo and flat or slightly concave from the umbo to the margins; the dorsal valve is 

 strongly and regularly convex. The description of the type species, Protorihis (Loperia) 

 dugaldensis Walcott, mcludes that of the subgenus, as it is the only species thus far known. 



The subgeneric name is given in recognition of the effective work of Mr. S. Ward Loper, 

 who collected the material representing this and many other Cambrian fossils in Cape Breton 

 and New Brunswick. 



Protorthis (Loperia) dugaldensis Walcott. 



^ Plate XCIX, figm-es 5, Sa-j. 



Protorthis {Loperia) dougaMensis Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 287-288. (Described and dis- 

 cussed as below as a new species.) 



General form rounded subquadrate. Hinge line straight and shorter than the greatest 

 width of the shell; cardinal extremities obtusely angular. Relative convexity of the valves 

 reversed as in Dinorthis pectinella [Hall and Clarke, lS92c, PI. V, figs. 28-31] of the Ordovician 

 fauna. Surface of the valves with strong, simple, rounded ribs that increase by interpola- 

 tion; fine concentric striae and lines of growth, that sometimes form ridges, cross the ribs, 

 curving in the interspaces and over the ribs; these ribs are more prominent and numerous 

 (30 to 40) on the dorsal than on the ventral valve; on the latter the ribs are broad, with narrow 

 interspaces, and about 20 to 25 in number; casts of the exterior and interior surfaces have 

 numerous fine, thickly set, elevated papiUse that appear as the casts of the punctse in the layers 

 of the shell; on some specimens the papUlse are cylindrical and elevated, wliich indicates that 

 they penetrated deep into the shell substance. All indications point to a punctate shell; in 

 all the material representing the species the shell has been removed by solution, only the casts 

 of the inner and outer surfaces remaining. 



The largest ventral valve in the collection has a length of 18 mm., width 25 mm.; dorsal 

 valve, length 22 mm., width 27 mm. 



Ventral valve elevated at the umbo and gently concave from the umbo to the front margin 

 and flat to the cardinal extremities. Area relatively low and overhanging the hinge line at an 

 angle of about 45°; it is divided midway by a broad delthyrium that is more or less closed 

 by a deeply concave plate, which is crossed by the transverse strise of growth of the area. The 

 concave plate appears to be identical in form and position with the free spondylium of the 

 genus Protorthis. A cast of the interior (fig. 5) shows very strong teeth and supporting dental 

 plates; the edges of the dental plates appear to be continuous with the concave plate or free 

 spondylium. 



Dorsal valve strongly and regularly convex, with a slight flattening of the median line 

 and sometimes a shallow sinus. Casts of the interior show a strong area, with a triangular 

 delthyrium; the umbonal cavity is strongly outlined by the crural plates so as to form a pseudo- 

 cruralium; crura well developed, with clearly defined tooth sockets back of them. No traces 

 of a cardinal process have been seen in ten specimens showing fine casts of the area, delthyrium, 

 and umbonal cavity. 



Olservations. — This fine species is unique among Cambrian brachiopods in having a 

 depressed, flat, or convex ventral valve and strongly convex dorsal valve; in its punctate 



