OBOLID^. 577 



plates had a projecting boss or tooth that articulated in a rude manner with the depressions 

 on the posterior, flattened, inner margin of the dorsal valve. If this interpretation is correct 

 the teeth and plates correspond to the teeth and dental plates of the articulate brachiopods 

 and the plates may mark the beginning of a spondylium. 



A number of interiors from Hudson, Wisconsin, show a concave plate ia the pedicle furrow, 

 the edges of which rise above the plane of the cardinal area; this plate is evidently the begin- 

 ning in Dicellomus of the pedicle tube, as shown in Oholella (PL LV, figs, le, If, and Ih). In 

 some shells the plate appears to extend into the shell beyond the anterior margin of the car- 

 dinal area and it then has the appearance of a small spondylium. I think, however, that it is 

 only the secretion of shelly matter about the pedicle and not connected with the development 

 of a spondylium; the latter is indicated by the extension of the toothlike processes already 

 referred to. 



The posterior lateral, transmedian, and anterior lateral muscles were undoubtedly 

 attached within the area of the composite scar (cl) (PI. LII). The visceral area and pos- 

 terior portions of the main vascular sinuses are well outlined, but no muscle scars or details of 

 the vascular sj^stem have been preserved in the material studied. 



Dorsal valve most convex back of the center. Apex marginal. The interior of the valve 

 shows shallow depressions in the flattened posterior margin (a) that suggest dental cavities 

 for the reception of the dental projections of the ventral valve. The central muscle scars 

 (h) are shown a little back of the center of the valve, where a low median ridge bifurcates 

 (PI. LII, fig. li). In some shells a sharp median septum is shown, also narrow main vascular 

 sinuses. 



Observations. — The reference to this species of the specimens from the siliceous shale at 

 Helena is rather doubtful, as the sheUs are compressed and distorted. (See PI. LII, figs. 4 

 and 4a.) 



Shells agreeing with D. politus in all points available for comparison occur in the basal, 

 probably Middle Cambrian, sandstone beneath the great limestone series of Arizona. The 

 material is abundant and preserved very much in the same condition as that from the Deadwood 

 formation of the Black Hills of South Dakota. 



In sandstones in the basal part of the Upper Cambrian Reagan sandstone in Oklahoma 

 a few shells were found that appear to be identical with D. politus. They have the elongate 

 form of that species, which is unknown in any other species of the genus. 



In the coUections made by the Fortieth ParaUel Survey there are specimens so closely 

 resembling D. politus that they are identified as such, although the dorsal valve is rather 

 transverse. 



Dicellomus politus differs from D. nanus (Meek and Hayden) and D. pedenoides (Whit- 

 field) in being more elongate, also in the details of the interior markings of the valves. From 

 D. appalachia Walcott it differs in interior markings, and, as specimens average, in being 

 more elongate. 



Formation and locality. — Upper Cambrian: (82b) "St. Croix sandstone" along the railroad track near Taylors 

 Falls, Chisago County; (97 and S97x) "St. Croix sandstone" at Reads Landing, foot of Lake Pepin, Wabasha County; 

 (339i) [Owen, 1S62, desc. of PI. IB'] sandstones of Fib of Owen' s section, below Mountain Island, in Mississippi Eiver, nearly 

 opposite the old mouth of Black River; and (98a) "St. Croix sandstone" at Marine Mills, on St. Croix River, Washington 

 County; all in Minnesota. 



(78a) "St. Croix sandstone" in the topmost quarry on Mount Washington, near Eau Claire, Eau Claire County; 

 and (98 and 98x) "St. Croix sandstone" near Eau Claire, Eau Claire County; (79s) "St. Croix sandstone" in bluff 

 near Hudson, St. Croix County; (328c) "St. Croix sandstone" at Alma, Buffalo County; (83) "St. Croix sandstone" 

 near Trempealeau, Trempealeau County; (100a) "St. Croix sandstone" at Ettrick, Trempealeau County; (80) "St. 

 Croix sandstone" 0.66 mile (1.1 km.) southwest of the railway depot, Menomonie, Dunn County; and (100) "St. 

 Croix sandstone" near Menomonie, Dunn County; all in Wisconsin, 



(128a) Shales on the Louisville and Nashville Railroad, 1.5 miles (2.4 km.) southeast of Cog Hill, near the line 

 between Polk and McMinn counties, Tennessee. 



62667°— VOL 51, pt 1—12 37 



