CHAPTER XIII. 



MESOZOIC FOSSILS. 



By T. W. Stanton. 



The Mesozoic fossils obtained in and near the Yellowstone National 

 Park and submitted to me for study include 78 species of invertebrates, of 

 which 31 are Cretaceous, 46 are Jurassic, and 1 is from beds of supposed 

 Triassic age. The number of species from a single horizon is not large 

 enough to be dignified with the designation "fauna," excepting, perhaps, 

 in one or two cases; yet the study of these fossils and the comparisons 

 made with known horizons have led to some general results that are worthy 

 of brief discussion. The subject will be treated by geological horizons, 

 and after reviewing the general considerations an annotated list of the 

 species with descriptions of new forms will be given. 



TRIASSIC. 



The Teton formation, of supposed Triassic age, yielded a few speci- 

 mens of a Lingula at a locality on the summit of Quadrant Peak. This 

 fossil resembles Lingula brevirostris M. and H., from the Jurassic of the 

 Black Hills, but in the absence of other fossils it should be given little 

 weight in determining the age of the beds. Linguloid shells are so slightly 

 differentiated that it would not be safe to distinguish, by them alone, even 

 between Paleozoic and Mesozoic. The determination of the age of this 

 formation must, for the present at least, rest on the evidence of stratigraphy 

 and lithology. The paleontologist can only say that the underlying beds 

 yield Carboniferous fossils, while the overlying formation has a well- 

 developed Jurassic fauna. 



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