186 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



region under discussion, nor lias it been found in the upper Fox Hills 

 strata of the Upper Missouri Eiver region. Its first appearance in tlie 

 Cretaceous rocks of the West seems to be at the base of the Fort Pierre 

 Group, iu the last-named region, and it apparently became extinct there 

 before the close of the Fox Hills period. In the uppermost strata of the 

 Fox Hills Group of that region it seems to be replaced by B. asper Mor- 

 ton, or an alhed species, but, as will be seen on a subsequent page, I 

 found it associated with Cardium speciosum and Mactra alta in the upper 

 strata of the Fox Hills Group in the valley of White Eiver, west of the 

 Eocky Mountains. 



39. Scaphites nodosus Owen. 



Some good examples of this species were obtained at Fossil Eidge, 

 but it was found at no other locahty in this district. In the Upper Mis- 

 souri Eiver region it seems to be confined to the Fort Pierre Group, and 

 it also seems to hold a similarly low position here. 



40. Ammonites . 



A fragment of an Ainmonites was found at the Little Thompson Creek 

 locality, which appears to ditt'er from any described species, but it is too 

 imperfect to base a satisfactory description upon. 



41. Placenticeras lenticulare Owen. 



This species was found in at least a recognizable condition at Fossil 

 Eidge, at the mouth of the Saint Vrains, and in the valleys of the Cache 

 a la Poudre and Little Thomi)Son Creek. It seems to range through the 

 whole Fox Hills Group, including the Fort Pierre division. 



42. Fish remains. 



The only vertebrate remains discovered in the Cretaceous strata of this 

 district were those of fishes, and which are very rare. At the mouth of 

 the Saint Vrains I found a single imperfect tooth ; and in the same strata 

 a few vertebrae and fragments of other bone and a few scales of teliost 

 fishes were discovered. Two or three similar vertebrae were also found 

 in the valley of Little Thomx^son Creek. 



The object of the foregoing lists of fossils and accompanying notes is 

 the presentation of ready means for the comparison of the fauna of those 

 Cretaceous strata of Eastern Colorado which I especially examined in 

 the season of 1877, with that of the equivalent strata of the Uj)per Mis- 

 souri Eiver region, the groux)ing of which by Hayden and Meek has become 

 typical in the paleontological history of the West. In the course of my 

 field investigations east of the Eocky Mountains in Colorado I also ex- 

 amined the Cretaceous strata of the Colorado and Dakota Groups ; but 

 as I only obtained a few fossils from them, and these being all quite 

 different from any of those of the Fox Hills Group already enumerated 

 in the foregoing lists, I defer a consideration of them to a subsequent 

 page. Besides those Fox Hills fossils already enumerated, I also ob- 

 tained others from near the towns of Golden and Morrison, Colo., but 

 as they present no additional facts of general application in the follow- 

 ing discussion, I shall consider them separately on a subsequent page, 

 iu relation to other facts of important but more restricted axDplication. 



For the i:)uri)Ose of avoiding confusion in the minds of those who shall 

 read this rex)ort, it may be weU to repeat the statement ab-eady made in 

 a foot-note, that the original grouping of the Cretaceous strata adopted 

 by Hayden and Meek for the Upper Missouri Eiver region, which is still 

 regarded as entirely appropriate there, has been so modified for Colorado 

 and the Territories adjacent as to include the equivalent strata of both 



