626 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



show the structure of the hinge nor other details of the interior that are 

 used as generic characters. 



The type is from the divide between Fawn Creek and Gallatin Valley. 

 The species is represented by nine other examples from Fan Creek Pass, 

 head of Gardiner ; saddle in ridge west of south branch of head of Gar- 

 diner; head of Fawn Creek northeast of Monument Peak, and Cinnabar 

 Mountain. 



Pleuromya subcompressa Meek. 



P]. LXXIV, figs. 8-11. 



Myaoites (Pleuromya) subcompressa Meek, 1873: Ann. Eept. U. S. Geol. Surv. Terr. 



for 1S72, p. 472. 1877: U. S. Geol. Espl. iOtli Parallel, Vol. IV, Pt, I, p. 136, 



PI. XII, figs. 6, 6a. 

 Myacites subcompressns (Meek) White, 18S0 : Ann. Eept. U. S. Geol. Surv. Terr, for 1878, 



p. 151, PI. XXXVIII, figs, ba-e. 



This most abundant species, which was originally described from 

 Weber Canyon, Utah, is represented by several hundred specimens, from 

 every Jurassic locality in the Park region at which fossils were collected 

 from the lower argillaceous limestone and shale. Almost every specimen is 

 more or less distorted, and every variation in form is seen that a thin-shelled 

 elongate species can be made to assume when embedded in soft strata and 

 subjected to pressure. In addition to these accidental distortions, it is evi- 

 dent that the species is naturally qrute variable in both form and sculpture, 

 some individuals being nearly smooth while others are marked by rather 

 strong concentric plications. Extreme variations approach the plicate 

 Pleuromya weberensis Meek on the one hand and the nearly smooth elongate 

 Pleuromya neivtoni Whitfield on the other. The extent and directions of vari- 

 ation are fairly well shown by White's figures above cited, though some of 

 these forms are slightly modified by pressure. 



Single specimens representing three or four extreme varieties could be 

 selected that if taken alone might be regarded as distinct species, but when 

 the attempt is made to classify the entire large collection coming from 

 practically one horizon and a limited area, it is found that none of the dis- 

 tinctions will hold good. 



The specimens figured show some of the principal variations in form, 

 and were selected from those apparently least modified by accidental dis- 

 tortion. They are from Fan Creek Pass, divide between Fawn Creek and 



