GEOLOGICAL SURVEY OF THE TERRITORIES. 459 



geuus from the Bitter Creek beds are not iu a condition to show the 

 aperture, beyond doubt, to possess tbe characters of Goniobasis. 



The entire absence among the fossils yet known from this formation 

 of Baculites, Scaphites, Ancyloceras, FfycJioceras, Ammonites^ Gyrodes, 

 Anchttra, Tnoceramtis, and all of the other long list of genera character- 

 istic of the Cretaceous, or iu part also extending into older rocks, cer- 

 tainly leaves its molluscan fauna with a strong Tertiary facies. Nor 

 can we quite satisfactorily explain this away on the ground that the 

 water in which this series of rocks was deposited partook too much 

 of the character of that of an estuary, to have permitted the existence 

 of any of these marine genera, because we do find iu it the genus Ostrea, 

 Anomia, and Modiola, which probably required water salt enough to 

 have permitted the existence of Inoceramus, Anchura, and Gyrodes^ if 

 not of some or all of the genera mentioned above. Indeed, at Coalville, 

 we find Inoceramus associated with some brackish-water types, and the 

 additional Cretaceous genera, Cyprimera, Anchura, Gyrodes^ &c., in 

 closely-associated beds. 



When we come to consider the invertebrate fossils yet known from 

 this formation, in their specific relations, we find all, with possibly tv/o 

 or three exceptions, new to science and different from those yet found 

 either at Bear Eiver, Coalville, or indeed elsewhere in any established 

 horizon ; so that we can scarcely more than conjecture from their 

 specific affinities to known forms as to the probable age of the rocks in 

 which we find them. Considered in this respect their evidence, how- 

 ever, is conflicting. Two of the species of Corhula, for instance, (0. 

 iropidopliora and G. widiferaj) are most similar to species found in the 

 brackish- water beds, at the mouth of Judith Eiver on the Upper Missouri, 

 that we have always considered Lower Tertiary ; though there are some 

 reason for suspecting that they may be Upper Cretaceous. A Corbicula, 

 both from the Black Butte and Point of Eocks localities, is even so very 

 nearly like G. cytJieriformis from the Judith Eiver beds, that I have 

 referred it doubtfully to that species. 



Again, the species Anomia gryphorhynclms, found so abundantly at 

 Point of Eocks, in the same bed with the above-mentioned Gorhicula 

 and Gorhula tropidophora^ so closely resembles a Texas Cretaceous shell 

 described by -Eoemer under the name Ostrea cmomiw/ormis, that I am 

 strongly inclined to suspect they may be the same ; though whether 

 identical or not, at least our shell is certainly not an oyster, as it has its 

 muscular and cartilage scars precisely as in Anomia, while its beak is 

 never marginal, and it has no ligament area. In all of these, (and 

 indeed in all other characters,) the Texas shell, as illustrated by Eoemer, 

 seems to agree precisely with ours, excepting that he represents it as 

 having only one central muscular scar, instead of three. In many of 

 our specimens, however, the two smaller of these scars are very obscure, 

 and might be easily overlooked. It is true he figures a nearly flat 

 valve without any byssal perforation, and a convex one, as opposite 

 valves ; and, if they are such, the shell would certainly not be aa 

 Anomia. Among a large collection of our shells, including thousands 

 of specimens, however, I have not yet seen a single perforated valve, 

 though they vary much in convexity, some of the valves being nearly 

 as depressed as the one Eoemer figures as the upper valve, suj^posing 

 it to be an oyster. If fhese depressed specimens in our collection are 

 opposite valves to the convex ones, then the shell would neither be an 

 Ostrea nor an Anomia, but would almost certainly fall into Morris and 

 Lycett's genus Flacmiopsis, which, so far as known in Europe, is a 

 Jurassic group. Consequently, if our shell should fall into that genus, 



