CAMBRIAN BRA CHIOPODA— WALCOTT. 315 



One of the types of D. narniK is the interior of a ventral valve, on 

 which there is a subtriangular depression that appears to have been 

 the path of advance of the area, on each side of the median space, in 

 Avhich the central muscle, middle, and outside lateral nuiscles were 

 attached. Another shell from the Big- Horn Mountains shows some- 

 thing of the same character. 



The area of the ventral valve is short, and divided midway by a 

 shallow pedicle furrow; area of dorsal valve unknown. 



The exterior of the shell appears to be smooth and slig-htl}' polished, 

 except for tine concentric strite and lines of growth and what appear 

 to be scattered puncta^. 



The average size of the adult shells is from 3 mm. to -l mm., the 

 length and width usually ])eing nearly the same. 



The exact horizon of the type specimens is unknown. The}' occur 

 in a somewhat friable, purplish sandstone, unlike any beds of the 

 Middle Cambrian Deadwood sandstone that I met with. The speci- 

 mens from the limestones of the Big Horn Mountains, etc., appear 

 to belong to the Uppper Cambrian terrane. 



Fonnathm andlocality. — Upper Cambrian, Shal}' sandstone, Black 

 Hills, North Dakota. Limestone above shales, about 10 miles south- 

 southeast of Bald Mountain, Big Horn Mountains; Gallatin limestone. 

 Crowfoot section, Gallatin Range, YelloM'stone National Park, W3^o- 

 ming. Limestone of the Little Rocky Mountains, west side of Dry 

 vCreek, below Pass Creek, Gallatin Valley. 



Two specimens of a shell apparently identical with D. nanus were 

 broken from a diamond drill core, taken from a fine sandstone 20 feet 

 above the La Motte sandstone, St. Francois County, Missouri. 



\ DICELLOMUS PARVUS, new species. 



General form ovate, with the ventral valve subacuminate and dorsal 

 valve broad oval to sul)circular. Valves moderately convex. Surface 

 of outer shell dai'k and polished; it is marked, when not abi'aded, by 

 tine, clearly defined, concentric strite and occasional lines of growth. 

 The largest ventral valve has a length of 2.5 nun. and a width of 

 2 mm. The shell is strong but not thick. Shell substance apparently 

 calcareo-corneouft. 



Ventral valve uniform!}' convex, except that the slopes toward the 

 cardinal margins are more abrupt than elsewhere; apex appears to be 

 marginal. The interior of the valve shows a short, low, median ridge 

 in the center of the visceral cavity; on each side, and a little in front 

 of the end of the median ridge are the trapezoidal areas for the attach- 

 ment of muscle scars; rather small, composite cardinal muscle scars 

 occur close to the cardinal margins. 



Dorsal valve somewhat less convex than the ventral; apex marginal. 

 The interior of the valve shows well defined composite cardinal muscle 



