WILLISTON. | The Kansas Niobrara Cretaceous. 241 
but a question of a few years when it will topple over and be de- 
molished. 
A. few miles west of Elkader in the Smoky Hill valley there is 
another conspicuous area of rocks resembling at a distance the 
ruins of many castles, and known as “Castle City,” a name given it 
by myself twenty years ago. 
Notwithstanding the general homogeneousness of the Ornitho- 
stoma beds in their lithological characters throughout their whole 
extent, there is a very distinct difference between the upper and 
lower horizons in the fossils. I have never made any special 
study of the invertebrates, and not very extensive collections, but 
’ from the thirty or more months of explorations I have become 
fairly familiar with the invertebrate fauna. But very few collec- 
tions have been studied by paleontologists, and I am sure that the 
field is rich for such explorations. 
INVERTEBRATES. 
Of the mollusca, Ostrea congesta is of very great abundance in 
the Rudistes beds, but much less common in the Hesperornis beds. 
They are found attached to other shells, and it may perhaps be in 
consequence of the fewness of large shells in the upper strata that 
their comparative rarity may be ascribed. Several species of 
Inoceramus are apparently found in all horizons, but the Haplos- 
caphas are abundant only in the lower horizons, and I never have 
found H. grandis or those allied to that species in the upper horizons. 
On the Smoky Hill river near the mouth of the Hackberry there 
are places where these shells can be gathered by the wagon load, 
often distorted, but not rarely in extraordinary perfection. A very 
thin shelled Inoceramid measuring in the largest specimens forty 
four by forty six or eight inches is not rare over a large part of the 
exposures. Invariably where exposed, as they sometimes are in 
their entirety on low flat mounds of shale, they are broken into 
innumerable pieces. For that reason, I have never known of one 
being collected complete, or even partially complete. Notwith- 
standing their great size the shell substance is not more than an 
elghth of an inch in thickness. Fragments of Rudistes are not rare 
—16 
