MESOZOIC FOSSILS. 605 



represented here. It has been recognized by means of its fossil plants at 

 Great Falls, Montana, and in the Black Hills, but its fresh-water mollusks 

 are almost entirely unknown, and the few that have been seen are entirely 

 different from these. All that can now be said concerning the age of these 

 fossils is that they come from a bed that is conveniently referred to the 

 Dakota on account of its stratigraphic position. I have named three of 

 the most common forms of gastropods from this bed, so that they may be 

 definitely referred to, although they are rather obscure and unsatisfactory 

 species. 



Colorado formation. — The marine Cretaceous beds on Snake River one-fourth 

 to one-half mile above the mouth of Sickle Creek may be directly correlated 

 with the upper part of the Colorado formation as it is developed on the Mis- 

 souri River near Fort Benton. The locality near Sickle Creek has yielded: 



Iuocerauras uudabuudus M. and H. Inoceramus flaccidus White. 



Inoceramus umbonatns M. and H. Baculites asper Mort. (?) 



Inoceramus acuteplicatus n. sp. Scaphites ventricosus M. and H. 



All of these, except the third and fourth, occur together in the upper 

 part of the so-called Fort Benton shales on the Missouri, and associated 

 with them are Inoceramus exogyroides M. and EL, I. deformis Meek, I. tenui- 

 rostris M. and H., Veniella mortoni M. and H., and Pholadomya 2)apyracea M. 

 and H., and a few undescribed species. 



This well-characterized zone was included in the Fort Benton shales 

 by Meek and Hayden when they gave that name to the "No. 2" of their 

 Cretaceous section, and they regarded all these dark shales near Fort 

 Benton as the equivalent of the shales underlying the Niobrara limestone 

 in Nebraska, Kansas, Colorado, and elsewhere. The fact is, however, that 

 the Niobrara also is represented by shales in this upper Missouri region, 

 and the fossils indicate that this zone is really the equivalent of the upper 

 portion of the Niobrara. The evidence for this statement rests on the 

 occurrence of several of the above species in the Niobrara limestone and 

 overlying shales of Colorado and in the equivalent Austin limestone of 

 Texas, and also on the absence of all these species except Veniella mortoni 

 from beds lower than the Niobrara in the same region and elsewhere. In 

 Colorado Inoceramus deformis is the most characteristic species of the Nio- 

 brara limestone. Recently Mr. G K. Gilbert has collected Inoceramus 

 umbonatus from shales in the Niobrara above the limestone near Pueblo, 



