nience of future reference, and the characterization of the strata from which it comes, I have applied the above specific name.

This shell, like those of the three species of Meekoceras which have already been described, is depressed-discoidal; the siphonal side rounded in the young and apparently a little angular in the adult, but the latter feature is not plainly shown in the examples; umbilicus open, but comparatively narrow; sides of the volutions flattened, those of the outer one of adult shells almost flat, or only slightly convex except near the outer and inner sides; the latter abruptly shouldered; involution very great in the young, but so lessening with growth that the outer portion of the living-chamber of adult shells does not embrace by more than onefifth of its own width that next within. The examples do not show the character of the sutures satisfactorily, but the lobes and cells are both seen to be constricted about the middle, the ends of the former oblique, but digitate, as in Meekoceras, and the latter plain, as in that genus and Ceratites. Surface conspicuously marked by numerous longitudinal, abruptly raised lines, which enlarge into ridges in the adult, and which are separated by spaces a little wider than themselves; being also apparently continuous from the young state to the fully adult, so that the ridges and spaces, especially the latter, increase in width with the growth of the shell.

This species is much larger than either of the other species of Cephalopods which are described in this paper; the extreme diameter of the coil at fully adult size being not less than 180 to 200 millimeters; and the vertical diameter of the outer portion of the living-chamber about 80 or 85 millimeters.

Position and locality.-Jura-Trias strata, member D of the foregoing section; at locality No. 1, Southeastern Idaho. Collected by Dr. A. C. Peale.

ARCESTES

-?-Three or four young examples only of this species were obtained by Dr. Peale from member D of the section at locality No. 1, shown on a previous page, all of which are too imperfect for specific determination. They are, however, so very like A. gabbi Meek, Vol. IV, Geol. Expl. 40th Parallel, p. 121, pl. x, figs. 6, 6a, and 6, as to leave upon the mind a strong impression of their identity.

ARCESTES?

-?-Associated with the foregoing, a single specimen of another species, also very young, was obtained, which differs from the last in having a much more open umbilicus, and in the proportionally greater transverse diameter of the volutions.

Art. VI.-Jura-Trias Section of Southeastern Idaho

and Western Wyoming.

By A. C. Peale, M. D.

In connection with Paleontological Papers No. 9, published in this Bulletin (Art. V, pp. 105-117) by Dr. C. A. White, in which a number of new and specially interesting species of fossils are described, I have thought it may be both interesting and useful to give briefly a few notes on the section of the Jura-Trias of the region from which these organic remains were obtained.

The rocks under consideration are exposed in considerable areas in the district assigned me for examination during the field-season of 1877, especially between the Wyoming and Portneuf Ranges. The former is in longitude 110° 48′ and the latter in approximate longitude 1120.

The region thus indicated is one of complicated folds and great displacements, and the Jura-Trias rocks, with the conformably underlying Carboniferous, enter largely into the structure of the mountains; entire ranges, indeed, being carved from them.

The Triassic Red Beds are found on the summit and on the western slopes of the Wyoming Range, in the former occupying a broad synclinal depression, and in the latter dipping westward into the valley of John Day's River. An immense fault extends along the eastern front of the Wyoming Range, giving it a monoclinal structure at this place, and when we cross the fold, which forms the western side of the range, we find another similar fault along the eastern side of the Salt River Range, which is parallel to the Wyoming Range, and only from eight to ten miles west of it. These faults are several thousand feet in extent.

In the Salt River Range, the Jura-Trias rocks are found entering into the complicated folds that form its central portions.

Twenty-five miles farther west, we reach the Blackfoot Basin, in which the Jura-Trias forms a large portion of the surface, the ridges having a comparatively low elevation, and the folds not being so greatly eroded as in the higher mountains. It is in this locality that the interesting collection of fossils, described by Dr. White, was found, and to which his paper is confined. The following is the general section of the JuraTrias, beginning at the top:

119

General Section of Jura-Trias in Southeastern Idaho and Western Wyoming.

5. Red and gray shales with green sandstones and irregular greenish arenaceous limestones at the top. Thickness, 700 feet.

4. Laminated limestones, blue at base, passing into gray at top, succeeded above by grayish, calcareous shales; many of the layers are probably arenaceous. Thickness, 800 feet.

3. Red quartzitic sandstones with shaly arenaceous and calcareous layers at the base of the section. These are probably the equivalent of the typical "Red Beds" of the Eastern Rocky Mountains. Thickness, 1.000+ feet.

2. Alternations of reddish and greenish sandstones and arenaceous and calcareous
shales with blue and gray limestones, generally laminated. Thickness, 3,000+
feet.
These with No. 1 are the beds of the section given in Dr. White's paper (as that
of locality No. 1). The beds are fossiliferous at four horizons, containing species
of a new Triassic? genus named Meekoceras by Professor Hyatt, together with
forms that have been heretofore regarded as of Jurassic age.

CARBONIFER- 1. Massive grayish-blue limestone, overlaid by quartzite and dark blue laminated OUS! limestones. Thickness, 1,200+ feet.

The thicknesses in this section are estimated, and when the region is more thoroughly studied may have to be somewhat modified.

The fossils described in Dr. White's paper came from No. 2, which I have called the "Meekoceras Beds," from the new genus named by Professor Hyatt. They are the following:

Meekoceras aplanatum White.

Meekoceras mushbachanus White.
Meekoceras gracilitatis White.

Besides these, the following were obtained:

Arcestes? cirratus White.

Arcestes? sp. ?

Arcestes? sp. ?

Terebratula semisimplex White.

Terebratula augusta Hall and Whitfield?

Eumicrotis curta M. & H.

Aviculopecten idahoensis Meek.

Aviculopecten altus White.

Aviculopecten pealei White.
Gervillia sp.?

No. 3 yielded no fossils.

No. 4 yielded the following:

Pentacrinus asteriscus.
Camptonectes bellistriatus.

Trigonia sp.?
Myacites sp.?

From No. 5 I obtained:

Belemnites densus.

Aviculopecten idahoensis?
Gryphæa sp. ?

Undetermined bivalves.

At no one point was the entire section, as given above, exposed. The evidence for the section is as follows:

Nos. 1 and 2 were determined by the section at and near Station 66, south of John Gray's Lake (section at locality No. 1 in Dr. White's paper).

Near Station 56 in the Salt River Range, about 28 miles east of Station 66, beds lithologically similar to those of No. 2 in the general section just given, and containing Aviculopecten pealei and Gervillia, were seen in position above limestones similar to those of No. 1, which here contained quantities of Productus multistriatus. On this ground I have referred No. 1 to the Carboniferous.

Above the arenaceous and calcareous bed (which correspond lithologically and paleontologically with No. 2 of the section) in the Salt River Range are the red sandstones (No. 3). They are also probably in place above the section of Station 66, as is evident from the following:

Descending the ridge leading southwest from Station 66, southwesterly dips are noted in the strata last seen (the upper ones of section at locality No. 1, White's paper). After passing through the timber, which conceals the remainder of the section, we come to red sandstones, in all respects like those of the Salt River Range, having a northeasterly dip. We have therefore crossed a synclinal.

These facts, therefore, appear to justify me in connecting the sections.

Nos. 4 and 5 are determined by sections made in John Day's Valley, where they rest conformably on the "Red Beds." The entire section is conformable. Above the "Belemnites Beds" is a quartzite followed by a series of shales and sandstones several thousand feet in thickness, which has been referred to the Cretaceous.

It is not my intention to enter into any paleontological discussion; but there are some points based on the paleontological contents of the section to which I wish to refer briefly.

Pentacrinus asteriscus was found at a number of localities, other than those of the section, throughout the district, and always in beds above the horizon of the Red Beds. The following were the associated fossils identified by Dr. White: