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eastward, and apparently at about the same angle. The best exposures of these rocks in this vicinity occur opposite the central portion of the city, where they are brought to view in a number of abrupt, quickly concealed ridges. These ridges trend northerly and southerly, and appear to be all constructed upon the sanie pattern, having on the west a steep, on the east a more gradual slope. Only the western faces are naturally exposed. This uniformity of structure is very striking, and there are reasons for believing that it has resulted largely from successive short, sharp folds in the strata, of which we have a fine example in the rocks east of Lansingburgh; but as nearly the whole district is covered with a thick sheet of drift, and the rocks bear evidence of extensive faulting, much further study will be necessary before it will be fully understood.

These ridges generally consist for the most part of course red and yellow weathering slates and shales, with occasional thin-bedded sandstones; but the most of them are supposed, and four of them are known, to hold subordinate limestone deposits. Of these deposits the two westernmost individually consist of a few courses of thick-bedded limestone, and of irregular, sometimes lenticular, sparry and frequently pebbly masses, varying from one to several hundred pounds in weight, imbedded in a coarse, dirty-looking arenaceous matrix: while the others form tolerably compact, even-bedded limestones, with an abundance of scattered black nodules, from twenty-five to thirty feet in thickness.

So far as investigated, these limestones have been found to be highly fossiliferous, though the fossils are usually in a very fragmentary condition. From two of them—one of the conglomerates and one of the even-bedded masses—the writer has made frequent collections during the last three years. With a single exception the sime species occur in both. Up to the present time they have yielded eighteen species, which are distributed as follows: Protozoa (Archeocyathus)...

1 species. Brachiopoda......

7 Limellibranchiatil..

1 Gasteropoda

1 Pteropoda (Ilyolithes).

2 Annelida (Salterella) .........

1 Crustacea.....

5 Total, 18

Of these, six-Obolella (Avicula ?) desquamata (Hall), 0. (Orbicula ?) crassi (H.), 0. (Orbicula) calata (H.), Metoptoma rugosa (H.), Theca trianguluris (H.), and Agnostus lobutus (H.)—were figured and described in the first volume of the Palæontology of New York in 1847, from this locality; and twoConocephalites (Atops) trilineatus (Emm.) and Olenellus (Elliptocephulus) asphoides (E.),* from Greenwich, Washington county. All the rest are new or undescribed. †

Desiring further information in regard to certain of these new species, I several months since wrote Mr. E. Billings, Palæontologist of the Geological Survey of Canada, at the same time giving him a list of the species in my possession from this quarter. In reply Mr. B. informed me that he was just engaged upon a collection of new fossils from the Lower Potsdam formation below Quebec, which he strongly suspected to be identical with my own: and on comparison it was found that fifteen out of the eighteen species from Troy were held by us in common, and shown to be perfectly identical. Such an unlooked-for result of course surprised us greatly. That the Lower Potsdam formation below Quebec, and the western portion of the Taconic series near Troy are of the same age, there seems now but little room for doubt.

Two very characteristic fossils of this formation are the opercula of two species of Ilyolithes, upon which I communicated a

These two species, to which great interest has long attached, were, until quite recently, supposed to be contined to an exposure of the - Black Slate" of Dr. Emmons, about two miles north of Bald Mountain, N.Y., where they were first discovered by Dr. Asa Fitch of Salem, N. Y., so long ago as the year 1844. Owing to the imperfec. tion of the specimens furnished by that locality, however, their true relations have long been considered doubtful among geologists. But the state of preservation in which they are now found in limestone leaves no longer a doubt as to their true affinities. Good specimens of these species are comparatively rare in the limestones at Troy, though fragments of large individuals of the Olenellus asaphoides are very common. I am indebted to Mr. Billings for having pointed out to me the specific identity of the Troy specimens with the stops and Elliptocephalus—an acknowledgment which was unintentionally omit. ted in this paper as originally published. As it is, however, about to be republished in the “ Naturalist and Geologist," I gladly embrace this opportunity to set the matter right.

† Unless one of them should prove identical with the species of Cypricardia figured by Emmons (American Geology, p. 113, plate 1, fig 1.)

note in the preceding number of this Journal. One of them was there described as a “minute, circular species, with four pairs of lateral muscular impressions and two smaller dorsal, all radiating from a point near one side;" the other as “larger, and like a Discina on the outside.” The former occurs quite abundantly in the Troy limestones, and is a very beautiful little object. It varies in size from a mere point to a diameter of three lines. Perfect specimens have a rich, polished appearance. The other occurs more rarely. As might naturally be expected, these rocks contain immense numbers of Iyolithes. Indeed, large portions of the limestone are often almost wholly composed of them.

Without doubt this formation in New York will yet afford many new species.* The even-bedded limestone east of Troy, to which especial attention has been given, its well as portions of the conglomerates, are literally loaded with fossils, and promise richiy to repay investigation for a long time to come. Their associated slates, shales and sandstones have as yet afforded no fossils. Near Lansinyburgh, however, where what is at present regarded as a lower member of the formation, consisting of heavy and thin-bedded gray

sandstones with interstratified black slates, is exposed, a few obscure Fucoids h:ve been found, but these rocks have been but imperfectly investigated. Neither the thickness nor precise eastern limit of this formation has yet been ascertained.

Troy, N. Y., May 24, 1871.

• These rocks have hitherto been referred, though with some doubt, to the Calciferous portion of the Quebec Group; but all modern investigations in our older strata have steadily pointed to their higher * antiquity; and it is simply justice to state that, by several geologists

besides those who have adopted Prof. Emmons' views of their age, this has long been suspected.

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Fig. 1. IIyolithes communis. 2. H. Americanus. 3. H. ? micans. 4. H. princeps. In these diagrams a ripresents the rate of tapering of the shell on the ventral side ; b, the transverse section (except in 3 b, which is the inner surface of an operculum enlarg d two diame. ters). The small figure in 3 a represents the apical portion of a specimen. N.B.—All these species vary slightly in the rate of tapering.

Genus HIYOLITHES, Eichwald. In the following description of new species of Iyolithes, I shall call the side of the fossil which is most flattened, or from which there is a projection in front of the aperture, “the ventral side.” Directly opposite is the “ dorsum." The lateral walls, whether consisting of two sloping planes, as in fig. 2, or rounded as in the other figures, I shall designate simply " the sides." The "width" of the aperture is the greatest distance between the two most projecting points of the sides. This is sometimes close to the ventral side as in fig. 2. The " depth" is the distance between the median line of the ventral side and the dorsum, and is at right angles to the width. That part of the ventral side which projects beyond the aperture is the “ lower lip.” The “ventral limb " of the operculum is that side which is in contact with the lower lip, when the operculum is in place, in the aperture. The “ dorsal limb" is the opposite side of the operculum, in contact with the dorsum. In some of the opercula there is a point around which the surface markings are arranged concentrically; this is the “nucleus."

The following species occur in the pebbles and boulders of a conglomerate which constitutes an important formation on the south shore of the St. Lawrence below Quebec. The age of the rock in which these pebbles are found, is not yet certainly determined, but it is, at all events, near that of the Potsdam.

H. COMMUNIS.—This species attains a length of about eighteen lines, although the majority of the specimens are from ten to fifteen lines in length. The ventral side is fiat (or only slightly convex) for about two-thirds the width, and then rounded up to the sides. The latter are uniformly convex. The dorsum, although depressed convex, is never distinctly flattened, as is the ventral side. The lower lip projects forward for a distance equal to about one-fourth or one-third the depth of the shell. In a specimen whose width is three lines the depth is two lines and ahalf.

The operculum is nearly circular, gently but irregularly convex, externally and concave within. The ventral limb is seen on the outside as an obscurely triangular, slightly elevated space, the apex of the triangle being situated nearly in the centre of the operculum. The base of the triangle forms the ventral margin. This limb occupies about one-third of the whole superficies of the external surface. The remainder, constituting the dorsal limb, is nearly fat, slightly elevated from the margin towards the centre. On each side of the apex of the ventral limb there is a slight depression, running from the nucleus out to the edge. On the inside there is an obscure ridge, corresponding to each one of the external depressions. It is most prominent where it reaches the edge. These two ridges meet at the centre, and divide the whole of the inner surface of the operculum into two nearly equal portions.

The surface of the operculum is concentrically striated. The shell itself in some of the specimens is covered with fine longitudinal striæ, from five to ten in the width of a line. The shell varies in thickness in different individuals. In some it is thin and composed of a single layer, but in others it is much thickened by concentric laminæ, and thus approaches the structure of a Salterella. There are also fine engirdling striæ, and sometimes obscure sub-imbricating rings of growth.

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