198 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



drecl feet below the bed of coal there, and the other about seven hundred 

 and fifty feet below it. The coal is embraced within the strata of the 

 Laramie Group, and is apparently not much more than one hundred feet 

 above its base, but no plane of demarkation between the two groups 

 has yet been satisfactorily recognized. IsTos. 6, 7, 9, and 10 of the fore- 

 going list were obtained from the upper fossiliferous horizon on Bear 

 Creek, and they are also all characteristic of the uppermost strata of 

 the Fox Hills Group as seen in the valley of the Cache a la Poudre and 

 at the mouth of the Saint Vmins, and the upper Bear Creek horizon is 

 therefore no doubt equivalent with those strata. The species represented 

 by the four foregoing numbers are discussed in the notes following the 

 lists of fossils collected in the district of the Cache a la Poudre and 

 Saint Vrains. The two species of Nucula represented by ISTos. 4 and 5 

 of the list were obtained from near Golden City by Captain Berthoud. 

 They also exist in the uppermost fossiliferous strata of tlie Fox Hills 

 Group in the valley of the Cache a la Poudre, but in the Upper Missouri 

 Eiver region iSTo. 5 at least ranges as low as the upper part of the Fort 

 Pierre Grouj). All the remaining species of the list that were found 

 in the Fox Hills strata of Bear Creek are from 750 feet below the coal. 

 The existence and association of the Cyrenaf hohnesi of Meek {=Mactra 

 holmesi White) and ScapMtes mandanensis ? at both the Ealstou Creek 

 and Bear Creek localities has already been stated and commented on. 



The three species oljtained from the Colorado Group at Bear Creek 

 are from the upper i^ortion, doubtless representing in part the Mobrara 

 Group or Cretaceous jSTo. 3 of the Upper Missoiui section. The speci- 

 mens of Ostrea congesta were found adhering to the shells of Inoceranius 

 deformis. The former is an abundant and widely-distributed species, but 

 the latter has never, to my knowledge, been found in the Upper Mis- 

 souri Kiver region, although it is common in the latitude of Colorado 

 and southward. Inoceramus problematicus, 'So. 18 of the list, does not 

 appear to range above the horizon at which it is found in the valley of 

 Bear Creek, east of the Eocky Mountains in Colorado, but in South- 

 western Wyoming forms that are undistmguishable irom this species 

 are found in strata of the Fox Hills Group. 



The discovery by Mr. Lakes of a fossil shell in the strata of the Dakota 

 Grou]3 has been already referred to. The following is his account of it 

 in a personal letter to me under date of June 21, 1878 : " To-day in ex- 

 ploring some rocks of the Dakota Group, I found in some finely lami- 

 nated drab shales about 100 feet below the usual ridge of sandstone which 

 so characteristically caps the Dakota hogback, the shell Avhich I for- 

 ward to you by to-day's mail. The shell was found in undoubted Dakota 

 rocks, a little north of the river Samt Trains." This is quite an unex- 

 pected discovery, and the specimen is the first Inoceramus that has, so 

 far as I am aware, been found in strata of the Dakota Group. From 

 the above remarks of Mr. Lakes, and a pencil-sketch which accompanied 

 them, it seems probable that the shell in question really came from the 

 upper layers of the Atlantosaurian beds of Professor Marsh. It has 

 been much compressed, and is too imperfect for specific identification, 

 and is perhaps identical with I. umbonatus Meek & Hayden, a Fort Ben- 

 ton Group species, but it has in its compressed condition much the 

 aspect of I. vanuxemi Meek & Hayden, from the Fort Pierre Group. It 

 is of a decidedly Cretaceous, and not Jurassic tyi^e, which fact has an' 

 interesting relation to the age of the Atlantosaurian beds of Professor 

 Marsh, if the specimen really came from them. 



The deposition of sediment which formed these beds seems to have 

 been continued without uiterruption or material change of character to 



