40G PllOCEEDINGS OF THE NATIONAL MUSE CM. 



Formaiion and locality. — Middle Cambriau, Coosa formation; in and 

 attached to the outer surface of siliceous nodules, Coosa Valley, Cher- 

 okee County, Alabama. 



Type.—Eo. 27319, U.S.N.M. 



OBOLUS (LINGULELLA) HELENA, new species. 



General form ovate, with the ventral valve obtusely acuminate, and 

 the dorsal valve rounded ovate. Valves moderately convex, as far as 

 can be determined from their condition of preservation, in the shales. 

 Surface of shell marked by rather strong lines of growth and very fine 

 irregular, radiating and concentric striiie that appear to inosculate, the 

 surface having something of the appearance of 0. {L.) ella; the charac- 

 ter of the surface markings of the inner layers and the interior of the 

 shell is unknown. So far as can be determined, the shell is rather thin 

 and formed of a thin outer layer and one or more thin inner layers or 

 lamelliie. A ventral valve 8 mm. in length has a width of 6 mm.; 

 another 7.5 mm. in length has a width of 6 mm. An associated dorsal 

 valve 7 mm. in length has a width of o mm. These variations in out- 

 line are due to considerable extent to distortion. 



As shown in the cast of the interior of the shell, the area of the ven- 

 tral valve is rather long and marked midway by a strongly defined 

 cast of a pedicle groove, and midway between that and the outer mar- 

 gin by a very distinct flexure line. The area of tlie dorsal valv(? is 

 rather long and quite distinctly marked on a cast of the interior. The 

 cast of the interior of the ventral valve shows a strong main vascular 

 sinus on each side of the visceral area, and in a cast of the dorsal valve 

 a slight median septum is indicated; also traces of the main vascular 

 sinuses. The only traces of the muscle scars observed are the anterior 

 laterals and a suggestion of the central scars of the dorsal valve. 



Observations. — This species is associated with 0. (L.) ella in the sili- 

 ceous shales near Helena, Montana, and what appears to be a similar 

 form occurs with the same vspecies in Big Cottonwood Canyon. In 

 form and surface characters it belongs to the group of which 0. (L.) ella 

 may be taken as a type. 



Formation and locality. — Middle Cambrian; dark siliceous shale in a 

 quarry li miles south of Helena, Montana. 



Type.— 'So. 273:iO, U.S.N.M. 



OBOLUS (LINGULELLA) INO, new species. 



Shell a little smaller than the average of the species of the subgenus. 

 General form ovate, with the ventral valve subacuminate and the dorsal 

 valve ovate in outline. There is some range of variation in the out- 

 line of the valves. The convexity of the valves is fairly strong, as 

 the shells are preserved in the somewhat shaly sandstones. Ventral 

 valves 7 ram. in length have a width of from 5.5 to G mm.; a dorsal 

 valve 5 mm. in width has a length of 5.25 mm. 



