INTRODUCTORY REMARKS. \ LIX 



The invertebrate remains from these beds were described in t lie joint 

 names of Meek and Haydenin the Proceedings of the Philadelphia Academy 

 of Natural Sciences in 1856* They belong mainly to the genera Ostrea, 

 Anomia, Unto, Corbula, Sphesrium, Corbicula, Campeloma, Viviparus, Planorbis, 

 Goniobasis, Bulinus, etc., all, with possibly the exception of one species 

 of Campeloma, being specifically distinct from forms found in the Fort 

 Union group. In addition to this, they have a decidedly older look; 

 that is, they are all more changed by the process of fossilization than those 

 of the Fort Union group, which are usually as fresh and unaltered in 

 appearance as merely bleached .shells of existing species. The presence of 

 two species of Ostrea in considerable numbers, and of an Anomia, in these 

 Judith River beds, also imparts to their fauna a more brackish-water aspect, 

 and even renders it probable that some of the beds were deposited in salt 

 water, and that the associated fresh-water- and land-types were carried into 

 the same by streams. 



The fact that fresh- and brackish-water- as well as land-types of shells 

 have been almost everywhere found to present close similarity to existing 

 types in rocks of all ages in which they occur, taken in connection with the 

 additional fact that the species from these beds have not yet been found in 

 any well-established horizon elsewhere, renders them of very little use in 

 determining the age of these beds. 



The vertebrate remains from them, however, which were referred by 

 Dr. Hayden to Professor Leidy, were found by that gentleman to present 

 decidedly nearer relations to Cretaceous than Tertiary types, some of them 

 being Dinosaurian related to the Iguanodon, Megalosaurus, etc. Hence he 

 was led to express the opinion that these deposits represent a formation "like 

 that of the Wealden ".f From these facts, and the direct association of 

 these isolated deposits with marine well-marked Cretaceous strata, the 

 precise horizon of which was not then known (since found, as already stated, 

 to belong to the upper part of the Fox Hills group), Dr. Hayden and the 

 writer were led to express the opinion that there might be "here at the 

 base of the Cretaceous system a fresh-water formation like that of the 

 Wealden ".J 



Dr. Leidy also described among Dr. Hayden's collections from the lowest 



* Proceed. Acad., 1856, VIII, 115. 



t Proceed. Acad., 18r>C, VIII, 72, 89, and 311. See also Trans. Am. Phil. Soc, March, 1859, 123 



X Proceed. Acad., 1856, VIII, 114. 



11— VII 



