UIKTACRmUS : ITS STRUCTUEE AND RELATIONS. 9 



been in saurians, fishes, and mollusks, it is hard to understand why the 

 Crinoids are not more abundant, and why they are so limited as to horizon. 

 The Ostrea congesta is always found in the same exposures with the Crinoid 

 layer, and mostly below it. This was the case at the original locality in 

 Utah, where the first specimen was found by Marsh. The huge Inocera- 

 mid shells of Haplascapha grandis are also found a little way below. The 

 Crinoids do not seem always to be confined to the same exact horizon, 

 although they are nearly so. At Locality No. 3 I found evidence of two 

 distinct layers in which they occurred at a vertical distance of two or three 

 feet apart. But their general position is in the upper division, or Hesper- 

 omis beds, of the Niobrara, and the place to search is about the top of the 

 blue chalk, and up into the yellow. 



Grinnell and Williston have shown that U. socialis was evidently gre- 

 garious, living in swarms, whose individuals were inextricably entangled 

 with one another by their arms and pinnules. This opinion is confirmed 

 by my observations. In all the localities where the Crinoids occurred I did 

 not find a single detached specimen. Every one of them was more or less 

 entangled with others. So far as I know, the species has never been found 

 in America, except where a colony of considerable number was indicated by 

 the formation of a calcareous plate out of their skeletons. Most of those 

 so far discovered were comparatively small; but the colony embedded at 

 Locality No. 1 must have contained an immense number of individuals, 

 since by far the greater part of the specimens were crushed into unrecog- 

 nizable fragments, and cemented into a very firm limestone in the thicker 

 portions. Erosion of the Niobrara Chalk beds has been most favorable 

 for the fossil collector. They are cut up by several streams and hun- 

 dreds of ravines, with innumerable little branches, producing miles and 

 miles of splendid exposures. I know of no formation where the possi- 

 bilities of a reasonably complete collection of its fossil remains are better 

 than in this. 



I mention this to emphasize the fact that the most careful search of 

 these exposures, over an area of ten by twenty miles, by Mr. Martin and 

 myself, and a greater area by Mr. Martin alone, has failed to disclose any 

 trace of the Crinoids except where they occur in the colonies as above 

 described. We looked especially for the detached plates, not only at the 

 same horizon for miles, but also in the strata above and below it. Other 

 fossils are abundant. The Ostrea congesta is everywhere, in immense num- 



