INTRODUCTORY REMARKS. LI 



in the Upper Missouri than those already mentioned, we have not much pre- 

 cise information ; but, from specimens found al several oilier isolated localities 

 in thai region, it is believed to be somewhat extended there at the base of 

 the Fort Union group. 



Professor Cope mentions some vertebrate types found by him that would 

 appear to indicate its existence in New Mexico. We have, however, so far as 

 known to'the writer, no evidence from invertebrate remains, of its extension 

 so far southward, though there is no known reason why it may not do so. It 

 can only be said that none of the Upper Missouri invertebrates characteristic 

 of this horizon have yet been brought in from any locality south of about the 

 latitude of the southern boundary of Montana, with one exception, mentioned 

 below, in Southern Wyoming. 



The exception alluded to above is Corbicula cytheriformis, M. & H., one 

 of the most common species of the Judith River group at the typical locality, 

 and near the mouth of Muscleshell River on the Missouri in Montana. In 

 1873, several specimens of a Corbicula, agreeing exactly with the above 

 species, were found by the writer and Dr. Bannister at the top of a very 

 extensive series of brackish-water lignitiferous strata at Black Butte station 

 on the Union Pacific Railroad, Wyoming. It. is also an interesting fact 

 that almost directly associated with this Corbicula, and species of Ostrea, 

 Viviparus, Goniobasis, etc., we found bones of a large Dinosaur ian, that has 

 since been described by Professor Cope under the name Agathaumas sylves- 

 tris, and referred by him to the Cretaceous epoch, or to an intermediate 

 horizon between the Cretaceous and the Lower Eocene. Yet, directly mingled 

 with these Dinosaurian bones, numerous leaves of dicotyledonous trees occur, 

 that Professor Lesquereux as confidently refers to the Eocene. It would be 

 unsafe, however, to refer these Black Butte beds positively to the horizon of 

 the Judith River group on the evidence of a single species of Corbicula, the 

 presence of lignite, and Dinosaurian remains; but it is certainly an interesting 

 coincidence that it should be found here at this remote locality under such 

 similar circumstances. 



In this connection, it is proper to explain that this Wyoming Lignite 

 group, which we have called the Bitter Creek series, is of very considerable 

 thickness, and includes a number of beds of lignite, or, more strictly speak- 

 ing, brown-coal, at some places quite extensive, and of good quality. On first 

 examining two new species of shells from a lower position in this series at 



