garded as a fresh-water shell, and is often found associated with other fresh-water forms, and also with Nuculana, which is now known only in marine waters. Its other associates are Corbula, Corbicula, and Anomia. Position and locality.-Laramie Group, about 400 feet above its base, Danforth Hills, Northwestern Colorado. The locality is about 10 miles northeastward from White River Indian agency.

ART. XXIX.-PALEONTOLOGICAL PAPERS NO. 7: ON THE DISTRIBUTION OF MOLLUSCAN SPECIES IN THE LARAMIE GROUP.

BY C. A. WHITE, M. D.

The term Laramie Group is here used to include all the strata between the Fox Hills Group of the Cretaceous period beneath, and the Wasatch Group (=Vermilion Creek Group of King) of the Tertiary period above. That is, it includes, as either subordinate groups or regional divisions, both the Judith River and Fort Union series of the Upper Missouri River; the Lignitic series east of the Rocky Mountains in Colorado; the Bitter Creek series of Southern Wyoming and the adjacent parts of Colorada; and also the "Bear River Estuary Beds", together with the Evanston Coal series, of the Valley of Bear River and adjacent parts of Utah. Strata of this great Laramie Group are known to exist in other large and widely separated districts of the western portion of the national domain, but only those above indicated are especially noticed in this paper.

So far as the brackish-water mollusca of the Laramie Group have yet been investigated, they have proved, with few exceptions, to belong to types represented by living mollusks of similar habitat; and the freshwater and land mollusks of that group of strata belong almost wholly, if not entirely, to types that are fully represented by living species. Therefore a mere similarity or even identity of molluscan types in the strata of the different regions just enumerated would not prove them to belong to the same epoch; but it is held that an identity of species does constitute such proof.

During the season of 1877 it was my good fortune to make considerable collections of fossils from all the forenamed regions except those of the Upper Missouri River. Study and comparison of my own collections with those made many years ago by Dr. Hayden from the Judith River and Fort Union beds in the Upper Missouri River region shows an intimate relationship to exist between the molluscan fauna of each of these series respectively. This fact is illustrated to some extent by the following table, which, however, includes only the species that have been discovered in the strata of more than one of the regions, or of the suborBull. iv. No. 3—12

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dinate groups, herein discussed. It is, therefore, by no means a summary of the invertebrate fauna of the Laramie period.

Table showing the Geographical Distribution of Species in the Laramie Group.

The underscore of the asterisk in the above table indicates the region in which the species thus designated was originally discovered. The double vertical line may be taken to represent the Rocky Mountains, or the great range, extending northward through Colorado, Wyoming, and Montana; the localities named on its left being east, and those on the right, west of those mountains.

The region indicated in the table as "South Platte Valley" embraces quite a large area east of the Rocky Mountains in Colorado, which is drained by the South Platte and its tributaries, and extends eastward from the base of the mountains out upon the plains, a known distance of 150 miles, and doubtless much further.

The Bitter Creek series, as here indicated, embraces all the strata that were included by Mr. Meek under the same designation in Hayden's Sixth Annual Report of the Geological and Geographical Survey of the Territories. Those of the well-known localities, Rock Springs and Black Buttes Stations, are both included in this series, and not regarded as separate, as they were in one of my former publications (Geology of the Uinta Mountains, Chapter III). The Yampa and White River Valleys are adjacent regions west of the Rocky Mountains, in Northwestern Colorado. The strata here included under the head of "Bear River Valley" are

those that have been frequently designated as the "Bear River Estuary Beds", and sometimes as the "Sulphur Creek Estuary Beds"; together with the coal-bearing series that is seen to rest upon them in the Valley of Bear River, northward from Evanston, Wyoming.

It will be seen that Ostrea wyomingensis is indicated with doubt as occurring in the Judith River Group. This reference is made because of the probable identity of Ostrea glabra Meek & Hayden, with 0. wyomingensis Meek, and the doubt is expressed because the proof upon that point is not entirely satisfactory. The former species, as identified in the Lignitic strata east of the Rocky Mountains in Colorado, is there found to be connected by associated intermediate forms with shells that cannot be distinguished from the typical forms of O. wyomingensis, and therefore no doubt is expressed upon that point as regards that region. This species is not only found in the strata of the other regions indicated in the table, but in various localities within the great Green River Basin west of the Rocky Mountains it is found to range through the whole series of Laramie strata, a thickness of not less than 3,500 feet. I am also a little in doubt as to the real identity of Campeloma multilineata in the Bitter Creek series; but all the other species embraced in the table are probably correctly identified. Not only has the Ostrea wyomingensis the great vertical range in the Laramie Group which has just been mentioned, but Anomia micronema, Brachydontes regularis, Melania wyomingensis, and probably other species also, have an equally great vertical range; embracing, in fact, the whole thickness of the Laramie strata in the great Green River Basin, which thickness probably reaches a maximum of 4,000 feet.

It is a well-known fact that the aggregate thickness of the Laramie strata east of the Rocky Mountains in Colorado is much less than it is in either of the other regions here named. But those eastern strata appear to represent the whole Laramie period, because they contain all the species just mentioned that are known to range through the whole series west of the mountains, where it has its maximum thickness, and they also contain certain species associated in the same layers that appear to characterize the Fort Union and Judith River beds respectively, in the Upper Missouri River region, and not there associated together in either.

The distribution of species in the Laramie Group, on both sides of the Rocky Mountains, is too conspicuously shown by the table to need com

ment.

In the foregoing discussion only the species that are common to the strata of two or more of the districts here discussed have been considered. Therefore, only the faunal relationships between these regions, and not their differences, are shown. To show the latter, a consideration of all the species yet discovered in the strata of this great group is necessary. The characteristics of all the known species of the districts named, except a part of those of Bear River Valley, are in har