LOWER CARBONIFEROUS FOSSILS. 523 



The ventral valve is nearly flat. Sometimes it is slightly convex, but 

 usually the umbonal region alone is convex and the outlying portions of 

 the shell are somewhat excavated. 



Both valves are often marked by concentric corrugations, due to unequal 

 growth, while old shells are sometimes plicated into loose and irregular 

 radiating folds or corrugations along the periphery. 



The generic identification of this form, as represented in the Yellow- 

 stone National Park, I can not regard as doubtful. The septum in the 

 ventral valve characteristic of Derbya, the only other genus with which it 

 could be confused, did not exist here. 



Ortliotlietes inflatus is somewhat larger than 0. incequaUs. The height 

 of the area is said to be one-third as great as its width, and three times the 

 width of the foramen. It is described as differing from 0. incequaUs in 

 having a much more ventricose dorsal valve and in the much greater height 

 of the area of the ventral valve, in which the foramen is about three times 

 as high as wide, while in that species it is much wider than high. The 

 stria? are also coarse and more elevated. 1 In the Yellowstone National 

 Park form the area is many times as wide as high and the foramen is very 

 slightly higher than it is wide. The dorsal valve is also much less inflated 

 than the corresponding valve fig-ured by Hall and Clarke (loc. cit., PI. da, 

 fig. 24) 



I have not identified 0. inflatus in the collection studied. 



Formation and locality: Madison limestone, near summit of ridge, 

 west end of Hunter Peak, Absaroka Range; limestone bluff north of 

 Little Sunlight Creek, Absaroka Range, 600 feet above stream; Arnold 

 Hague. East side of Gallatin River, west of Electric Peak; divide 

 between Gallatin River and Panther Creek, Gallatin Range; east face of 

 Antler Peak, Gallatin Range; saddle west of Antler Peak, Gallatin Range; 

 amphitheater west of Bannock Peak, Gallatin Range, bed 26; amphitheater 

 east of same, bed 28; W H. Weed. Crowfoot Ridge, top of bed 25; 

 J. P. Iddings and G. M. Wright. Same, bed 26; lower part of bed 27; 

 upper part of bed 27; beds 28, 29, 31; J. P. Iddings and W. H. Weed. 

 South of Forellen Peak, Teton Range; northwest slope of same; S. L. 

 Penfield. West of Antler Peak, Gallatin Range; north of Bighorn Pass, 

 Gallatin Range; A. C. Gill. Crowfoot Ridge, Gallatin Range; top of bed 



'White and Whitfield, loc. cit., p. 293. 



