INTRODUCTORY REMARKS. LIU 



In commenting on these facts, the author remarked, in Dr. Hayden's 

 Sixth Annual Report, page 460, "thai the occurrence of this last-mentioned 

 species [the form similar to Viviparus trochiformis] here, along with a 

 Cretaceous type of Reptilian, and a Corbicula apparently identical with C. 

 cytheriformvi of the Judith River brackish-water beds, together with Corbulas 

 very closely allied to Judith River species, at lower horizons in this series, 

 and the occurrence of some vertebrates of Cretaceous affinities at the Judith 

 River localities, would certainly strongly favor the conclusion, not only that 

 this Judith River formation, the age of which has long been in doubt, is also 

 Cretaceous, but that even the higher fresh-water Lignite formation at Fort 

 Clarke [Fort Union group], and other Upper Missouri localities, may also 

 be Upper Cretaceous instead of Lower Tertiary." Immediately after this, 

 however, he added the following remark: 



"That the Judith River beds may be Cretaceous, I am, in the light of 

 all now known of the geology of this great internal region of the continent, 

 rather inclined to believe. But it would take very strong evidence to con- 

 vince me that the higher fresh-water Lignite series [Fort Union group] of 

 the Upper Missouri is more ancient than the Lower Eocene." 



Since the above was written, Professor Powell and Dr. White have been 

 so fortunate as to find, at Black Butte station, several nearly perfect speci- 

 mens of the shell, the upper volutions of which are so much like those of 

 Vwiparus trochiformis, and these show that its lower turns develop two or 

 three rows of nodes, thus removing it very decidedly from near relations to 

 V. trochiformis, and placing it in the distinct, but analogous fresh-water genus 

 Tulotoma* 



As we have no evidence yet of the occurrence of any other Fort Union 

 group species in the Bitter Creek series, it will readily be seen that we at 

 least still want the "very strong evidence" that it was stated would be 

 required to satisfy the author that the Fort Union group would possibly have 

 to be referred to the Cretaceous in case the Bitter Creek series should prove 

 to belong to that epoch. 



On the contrary, not only from the presence in these beds of Dino- 

 saurian remains, and one species of Corbicula undistinguishable from the 

 Judith River C. cytlieriformis, with one or two species of Corbula cry like 

 forms from that group, but aP.o from the general state of preservation ol all 



* ii has since been described by Dr. White under the name T. Thompsoni. 



