NO. 1395. CAMBRIAN BRACHIOPODA—WALCOTT. 269 



projects slightly. Dorsal valve slightly less convex than the ventral; 

 beak small, scarcely projecting beyond the hinge line. 



Surface marked b}'^ bifurcating, radiating costa?, that vary on shells 

 of similar size from 16 in the space of 5 mm. to 3 in the same space. 

 This variation is shown in the specimens from Texas, Wisconsin, and 

 Wyoming. In well preserved specimens very fine, radiating, raised 

 strise occur both on the costai and on the intervening depressions. 

 These are shown on the casts of the shells from the St. Croix sand- 

 stone of Winfield, Wisconsin, and on the larger shells from the lime- 

 stones of Indian Territory and the Yellowstone National Park. 



The interior of the ventral valve shows a slightly raised, rather 

 small tripartite umbonal cavity beneath the umbo, which is the only 

 trace of interior markings of this valve observed. The interior of the 

 dorsal valve has a slightly elevated area upon which occurs a narrow, 

 short median septum. The crural plates are also well shown. In 

 casts of the interior from the St. Croix sandstone of Wisconsin the 

 dental lamella? of the ventral valve are finely shown, and in the dorsal 

 valve the median septum and crural plates. 



This is one of the most variable shells that occur in the Cambrian 

 fauna. Its range of variation is such in all of the widely separated 

 localites in which it occurs that one would scarcely hesitate, if in pos- 

 session only of the extremes, to identify two well-marked species. 

 The variation is not only in the radiating costse, but also in the gen- 

 eral form of the shell. This variation is expressed in three varieties 

 that receive names, and others might be designated if more minute 

 variations were given consideration. 



Formation and locality. — Upper Cambrian. St. Croix sandstone, 

 Red Wing, Minnesota. Four miles north of Reedsburg and at Win- 

 field, Wisconsin. 



Near base of Upper Cambrian, on ridge eight miles east of Yellow- 

 stone River, three miles north-northeast of Mount Delano, Park County; 

 Gallatin terrane. Crowfoot section, Gallatin Range, Yellowstone 

 National Park. It also occurs at a slightly lower horizon on the south 

 side of the Gallatin Valley, and specimens were collected farther to 

 the north by Dr. A. C. Peale, opposite the mouth of Pass Creek, in 

 the Gallatin Valley, Montana. 



Upper division of Reagan formation, NW. i sec. 1, T. 2 S., R. 1 E. 

 West side of Honey Creek, near SE. corner sec. 35, T. 1 S., R. 1 E. 

 Ardmore quadrangle; northwest extremity of Arbuckle Mountains, 

 about 4 miles east of Homer, Indian Territory. 



PLECTORTHIS REMNICHA SULCATA, new variety. 



This variety is founded on the strongly sulcate dorsal valve. The 

 ventral valve shows only a slight flattening of the median portion. 



Formation and locality. — Upper Cambrian. St. Croix sandstone, 

 Winfield, Taylors Falls, and four miles north of Reedsburg, Wisconsin. 

 Proc. N. M. vol. xxviii— 04 18 



