bly enlarged, composing much the greater part of the shell, almost or quite regularly rounded from the suture to the umbilicus; suture well defined, and rendered still more conspicuous by the convexity of the volutions; umbilicus very small and deep; aperture short, subovate or subcircular, obtusely angular at its distal side; a little straightened by contact with the next volution between that angle and the umbilicus, and elsewhere almost regularly rounded.

Surface smooth, almost polished, but marked by very fine lines of growth.

Length from front to apex 18 millimeters; breadth of body-volution 18 millimeters.

This shell is proportionally shorter than any other species of the genus known to me, but it seems to possess all the characteristics of Viviparus. Its outer lip has the usual straight margin, but its umbili cus is a little more open than usual, and the inner lip not reflexed, but almost continuous in its curvature with the outer lip.

Position and locality.-Laramie Group, Crow Creek, Colorado, 10 miles above its confluence with South Platte River, Northern Colorado.

Viriparus coucsi (sp. nov.).

Shell very large when fully adult; volutions six or seven, convex, the distal side of the last one especially rounded abruptly in to the suture, giving it a somewhat shouldered aspect there, while the outer side is broadly convex and sloping gently forward and inward; suture deeply impressed, the apparent depth being increased by the great convexity of the volutions. Surface marked by the ordinary lines of growth, no revolving marks of any kind being detected. The lines of growth indicate that the margin of the outer lip was nearly straight, as is usual with species of this genus, and which character distinguishes it from Campeloma. Inner lip thickened, and reflexed at the proximal or anterior end, but not covering the umbilical fissure there, which is moderately large. The precise shape of the aperture is unknown, but it is probably ovate.

No entirely perfect examples have been discovered, but the largest one yet obtained would, if perfect, measure about 65 millimeters in length; full width of body-volution 38 millimeters.

This species is described by Meek in vol. iv, p. 181, pl. 17, fig. 15, King's United States Geological Survey of the Fortieth Parallel, and referred to the genus Campeloma, but not specifically named. The numerous specimens, however, that have been obtained from the typical and other localities show that the species possesses the true characters of Viviparus. This species is distinguished from all others of the genus known to me in American strata by its great size, and there are few other species with which it is in any danger of being confounded. From V. paludinæ formis Hall, it differs in its more robust form, in the greater convexity of its volutions and the abrupt rounding of their distal side, and in the presence of a comparatively large umbilical fissure.

Position and locality.-Laramie Group, Valley of Bear River, seven miles northwestward from Evanston, Wyoming, and at several points. in the vicinity of Mellis Station, Union Pacific Railroad, near the confluence of Sulphur Creek and Bear River. It is associated with Campeloma macrospira Meek, Unio vetustus Meek, and other fresh-water mollusks, as well as many brackish-water species.

Genus ODONTOBASIS Meek.

Odontobasis? formosa (sp. nov.).

Shell rather small; spire equal to about one-half its entire length; volutions about six, the body one inflated and the distal ones moderately convex, the distal part of each somewhat shouldered, and marked there by numerous small longitudinal folds that become obsolete toward the proximal part; these longitudinal folds are less distinct upon the body. volution than upon the distal ones; upon the latter also there is a small revolving furrow near to, and upon the distal side of the suture, giving those volutions a slightly constricted aspect, but which constriction does not extend upon the body-volution.

Surface apparently marked only by lines of growth, with the excep tions already mentioned, and some revolving ridges or lines upon the proximal side of the body-volution, near the beak.

Length 12 millimeters; breadth of body-volution 7 millimeters.

Only one specimen of this species has been discovered, and this is a somewhat distorted cast from the reddish shales of the Laramie Group near its base. Neither the aperture nor the extremity of the beak is shown in the specimen, and I am not entirely satisfied that it belongs to the genus Odontobasis, but its general aspect and observable characters favor that reference, although it bears considerable resemblance to Admetopsis Meek, from the Cretaceous strata at Coalville, Utah. Perhaps a sufficient reason for referring this shell provisionally to Odontobasis is the fact that a species of that genus is already known in the Laramie Group, while no other genus is yet known there to which it could be confidently referred. Of the three other species of Odontobasis yet known, two are from the Fort Pierre Cretaceous Group, a true marine formation, and one from the Laramie Group, near Point of Rocks Station, Union Pacific Railroad, Wyoming, a brackish-water formation, and which is there associated with Goniobasis insculpta as well as Ostrea and Anomia. The genus Admetopsis is not yet known to exist in the Laramie Group, nor unassociated with true marine forms.

When other specimens shall have been discovered, it may prove that the description should be somewhat modified, but it is doubtless quite sufficient for the identification of the species.

As a rule, the molluscan remains of the Laramie Group indicate a brackish condition of the waters in which they lived. This species is associated with Melania wyomingensis Meek, which is necessarily re

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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