318 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



interior of the valve shows shallow depressions in the flattened pos- 

 terior margin that sug'gest dental cavities for the reception of the den- 

 tal projections of the ventral valve. The composite muscle scar prob- 

 ably formed the point of attachment of the posterior muscles, trans- 

 median, outside, and middle laterals. The central muscle scars are 

 shown a little back of the center of the valve, where a low median 

 ridge bifurcates. In some shells a sharp median septum is shown; also 

 narrow main vascular sinuses. 



Ohscrvations. — The specimens of this shell from Wisconsin, Minne- 

 sota, and North Dakota occur in fine-grained, more or less friable 

 sandstone; those from Montana in a compact limestone in the Gallatin 

 Valley and a siliceous shale at Helena. The reference of the speci- 

 mens from Helena is rather doubtful, as the shells are compressed and 

 distorted. 



The ])eds in which the specimens are found at Taylors Falls, Minne- 

 sota, are said to be Upper Cambrian. If this is correct D. politus 

 ranges from the Middle to the Upper Cambrian. Shells agreeing with 

 D. j)<dit/(.'< 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 sandstone of the 

 Black Hills of South Dakota. 



In the sandstones at the base of the Middle Caml>rian Reagan forma- 

 tion 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 collections made b}' the fortieth parallel survey there are 

 specimens so closel}^ resembling I). polltuH that they are identified as 

 such, although the dorsal valve is rather transverse. 



DlcelJohnis 2>ol/ti(!< differs from Ij. nanns and D. pectenoldes in being 

 more elongate; also in the details of the interior markings of the valves. 

 From D. appahiclihi it differs in interior markings and, as specimens 

 average, in ])eing more elongate. 



ForiHdt'ion und locality. — Middle Cambrian. St. Croix sandstone, 

 middle ))eds of the sandstone, at Eau Claire, Wisconsin. Deadwood 

 sandstone, 9 miles west of Custer City; head of Red Water Canyon 

 and several other localities in the Deadwood standstone of the Black 

 Hills, South Dakota. (lallatin limestone, near Gallatin, and also in 

 Silurian shales with (Jholns {Lingulell(() helena and 0. ( Westo'ria) ella., 

 on the southern slope of Mount Helena at Helena, Montana. Basal 

 sandstone. Ash Creek, Pinal Count}^ Arizona. Basal sandstone of the 

 Reagan formation, Wichita Mountains, 11 miles northwest of Fort 

 Sill in the SW. i sec. 17, T. -t N., R. 12 W., Oklahoma Territory. 



Upper {'.) Candjrian. Up|)er beds of St. Croix sandstone at Taylors 

 Falls, Minnesota. 



