170 REPORT UNITED STATES GEOLOGICAL SURVEY. 



remarks under the head of Xo. 1, Anomia micronema^it "will be seen that 

 that species also was discovered in the same shaft, but at a depth of 45 

 feet ; that is, in a layer beneath the one in which the Gorbicula here 

 discussed was found. The layers which are respectively characterized 

 by these two species hold the same relative position at both the Crow- 

 Creek and Bijou Creek localities. This fact is without important signifi- 

 cance, so far as the vertical range of the different species alone is con- 

 cerned, at least that of the Anomia, the vertical range of which is known 

 to embrace almost the entire thickness of the Laramie Group west of the 

 mountains ; but it has much signiiicance as indicating great uniformity 

 of the conditions that affected deposition of sediment during the Lara- 

 mie period in the region which now constitutes a portion of the great 

 plains adjacent to the east base of- the Eocky Mountains in Colorado. 

 This subject, however, will be further discussed upon following pages. 



!No. 14. Corbula subtrigonalis Meek & Hayden. 



Dr. Hayden obtained the type-specimens of this species from the Ju- 

 dith River Group of the Upper Missouri River region. It is described 

 and figured in vol. ix of the United States Geological Survey of the 

 Territories. Like all species of this genus, it is very variable in surface- 

 markings and outline, but there seems to be no reason to doubt the iden- 

 tity of the specimens found at the Crow Creek locality with the species 

 above named, which is regarded as identical with G. perundata of the 

 same authors, and which is associated in the same strata in the Upper 

 Missouri River region. Neither have I any serious doubt of the identity of 

 this species with both G. tropidophora Meek and G. crassitellifoymis Meek, 

 both originally described by him from specimens obtained from the Lar- 

 amie strata in the valley of Bitter Creek, Wyoming. The latter, accord- 

 ing to Meek, was also found associated with 'So. 11 in the same shaft, 

 200 miles east of Denver, Colo. The differences between these forms 

 that have received different specific names are believed to be not greater 

 than the range of properly recognized interspecific variation under dif- 

 ferent conditions of environment. Several of the examples discovered, 

 not only in the valley of Crow Creek, but also in localities west of the 

 Rocky Mountains, are much larger than either of the Upper Missouri 

 River forms, or than any known examples of G. tropidophora, but this 

 is believed to be only a variation due to conditions of environment, 

 probably in this case a difference of saltness of the waters. 



]STo. 15. Bulimis disjunctus White.* v 



This species was obtained from jSTo. 5 of the Crow Creek section, and 

 specimens were also obtained from the coal-bearing series of the upper 

 part of the Laramie Group at the Almy coal-mines, near Evanston, 

 Wyo., that appear to belong to this species, but they have the spire 

 somewhat shorter. 



IsTo. 16. Bulinus subelongatus Meek & Hayden. 



This species was originally discovered by Dr. Hayden in the strata of 

 the Judith River Group of the Upper Missouri River, and it was also 

 recognized among some collections made in that region by Professor 



* This species has not hitherto heen described. It closely resembles B. elongatus 

 Meek & Hayden, but the spire is more elevated, and consequently the body-volution 

 is proportionally smaller. Besides this, the anterior half of the callus which forms the 

 inner lip is not appressed against and adherent to the body, whereas the posterior half • 

 is, and as the whole of it is in other species of Bulinus, but it is deflected or disjoined 

 so as to leave a kind of umbilical space between it and the body of the shell. This 

 peculiarity is not accidental, as„ was at first supposed, but it was observed in all the 

 specimens, young and old, of both the localities at which it was obtained. 



