206 EEPORT UNITED STATES GEOLOGICAL SUEVET. 



souri Biver region, and it also ranges low in the strata of the Fox Hills 

 Group of Eastern Colorado. In view of the high position of these fossils, 

 it seems probable that the examples in question belong rather to I. per- 

 tenuis than to I. vanuxemi. 



6. Niicula planimarginata Meek & Hayden. 



Several examples of this species were found here, all imperfect, but 

 all showing characteristic features. It is always found to range high in 

 the Fox Hills strata of Colorado. 



7. Thetis ? circularis Meek & Hayden. 



Two or three imperfect examples were found, which seeni, from their 

 external characteristics, to belong to this species, although they are all 

 smaller than the type specimens. The latter also have been found only 

 in the Fort Pierre Group of the Upper Missouri Biver region, while 

 those in question hold a much higher position. 



8. Teredo f 



These examples consist only of some borings in pieces of fossil wood 

 which have become filled with stony material ; but they seem without 

 doubt to have been made by a species of Teredo or an allied mollusk. 



9. Anisomyon cenirale Meek. 



A single example only of Anisomyon was found with the other fossils 

 of this collection. It doubtless belongs to this species, although it shows 

 a greater than the usual number of the irregular radiating grooves 

 which characterize most of these forms. Figures of Meek's types of this 

 species are given in another part of this volume. 



10. Bacidites ovatus Say. 



In the notes upon this species on a previous page it was stated that 

 its vertical range was not known to extend, either in the upper Mis- 

 souri River region or in Eastern Colorado, to the uppermost strata of the 

 Fox Hills Group. At the locality in question, however, as well as at 

 another locality also west of the mountains, in the valley of White Biver, 

 presently to be mentioned, it ranges not only into the uppermost strata 

 of the Fox Hills Group, but it is there associated with species that 

 especially characterize the uppermost strata of that group elsewhere. 



11. ScapMtes nodosus Owen. 



Four or five examples of this species were obtained at the locality here 

 discussed, one of which possesses all the characteristics of the variety to 

 which Mr. Meek gave the name of plenus. The others are by natural 

 growth so much compressed laterally that they might be readily mis- 

 taken for a separate species. They appear, however, to possess the es- 

 sential characteristics of the form to which Mr. Meek gave the varietal 

 name of quadrangular is, except that they want the flattened periphery, 

 bordered by numerous small nodes, which characterizes his type of that , 

 variety. 



While discussing the fossils of the Fox Hills Group, which were col- 

 lected east of the mountains in Colorado, it was shown that certain of 

 the species characterize certain horizons in the great group. For ex- 

 ample, certain species that are found only in the Fort Pierre Group of 

 the Upper Missouri Biver region, are found only at a correspondingly 

 low horizon in the consolidated Fox Hills Group as it is developed in 

 Eastern Colorado. Also, that the species which characterize the upper- 

 most Fox Hills strata in one of those regions are, as a rule, equally 

 characteristic of corresponding strata in the other. While these facts 



