group, in accordance with Hisinger's list of Swedish rocks as given in his Lethæa Suecia in 1837, and not as he had previously given it. But the discovery in 1860 of the Point Lévis fossils enabled Mr. Billings to prove that the rocks of the Quebec group must be placed near the base of the Lower Silurian series instead of at its summit, and it thus became necessary to discover some other interpretation of the physical structure than the one suggested by the visible sequence of the strata.

Although there may be difficulties in regard to detail, the interpretation given in my letter to Mr. Barrande of the 31st December, 1860, will, I am persuaded, turn out to be the right one. Prof. Emmons long ago asserted that the rocks in question in Vermont were older than the Birdseye and Black River formation. In this I now agree with him; while however his interpretation of the structure would make them all older than the Potsdam sandstone, mine would not. But whatever the value of my present interpretation, it might have been some time before I should have been urged to lock for it, had it not been for the palæontological skill which Mr. Billings brought to bear on the question. I am, dear Sirs, very truly and respecfully yours, W. E. LOGAN.'

To the above I shall add two quotations from the last letter I received from Dr. Emmons on the subject of the Taconic system. He was, at the date of this letter, State Geologist of N. Carolina : "RALEIGH, Feb. 5, 1861.

MR. E. BILLINGS:

"MY DEAR SIR,-I am much obliged to you for your favor of the 30th inst., and especially for the opinions and kind regards which you express. Be assured they are highly appreciated, and the more so seeing that they are rare. I had for years past looked upon the subject with a kind of indifference, until you had expressed to Col. Jewett, opinions favourable to the existence of the lower rocks I had contended for; not indeed that I had any misgivings of the truth of the position I had taken, for that would be impossible from all I had seen, provided there were truth in geology, and that the department were founded on principles. But the real difficulty has always been that geologists would not look at the question at all."

"Be assured that I fully appreciate your kind aid in the matter of the Taconic system, for I think I should have gone down to the grave before it had been acknowledged, except for your active, intelligent, and disinterested labours in the cause."

"Yours truly,

"E. EMMONS."

FIG. 1. Dragram of the interior of the ventral valve of a specimen, supposed to be a small individual of O. Canadensis; bb, the two large sub-central muscular impressions; dd, the groove under the area; e, enlargement of the same; g, the pedicel groove in the area, on each side of which is a smaller oblique furrow; r, the ridge in front of the muscular impressions.

2. Interior of a dorsal valve; a, the area; c, the pair of small scars in front of the two larger. The other letters, the same as in Fig. 1. 3. Dorsal view of the original specimen.

4. Side view of the same.

5. Ventral view of the same.

A short notice of this genus was published in the last number of this journal, Dec., 1871. I now propose to extend the des cription, so far as our present material will admit.

(All the figures in this paper are of the natural size.)

Genus OBOLELLINA, Billings.

Generic characters.-Shell, unarticulated, ovate or orbicular, smooth or concentrically striated. Area of the ventral valve with a median groove, on each side of which there is, sometimes, an additional furrow. In the interior of this valve there are two large, ovate, or sub-rhomboidal muscular impressions. They are situated near the centre, but usually (for the greater part) in the posterior half of the shell. They are sometimes obliquely striated or grooved, or obscurely reticulated by both transverse and longitudinal striae. Close under the area there is a fine, but distinctly impressed groove, which curves outwards and forwards, outside of the muscular scars for a greater or less distance towards the front margin. There appears to be an enlargement of this groove, just under the peduncular groove of the area, on the median line, as if for the attachment of a muscle. The large scars are bordered anteriorly, by an elevated margin, which is prolonged forwards, along the median line, in a more or less prominent ridge; this ridge varies greatly in the amount of its developement, in different individuals of the same species, being sometimes almost obsolete.

The area of the dorsal valve varies greatly in size in the different species, and is either flat or with a triangular elevation under the beak like a pseudo-deltidium. Beneath the area there is a fine groove, which curves outwards and forwards, as in the ventral valve, with a similar enlargement in front of the beak. There are two large, ovate, sub-central muscular impressions, with a smaller pair in front of, or between them. These latter are situated on or close to the median line, and usually appear as a single scar, but in some specimens are distinctly divided into two, by a longitudinal ridge. Their form varies in different individuals of the same species. The muscular impressions are margined, anteriorly, by an elevated border, which is extended forwards as an obscure ridge, a greater or less distance towards the front.

In the original notice it is stated, that this genus has no cavi. ties in either valve. This holds good for all the specimens of O. Canadensis of which the interior has been seen. In O. Galtensis, however, while some of the specimens have no cavities, in others, as is shown by the casts of the interior, there is a small one extending a short distance under the larger muscular scar on each side in the ventral valve. In one of our specimens there is a short cone, half-a-line in length, on the edge of the cast of the cavity. No

It thus

cavities have yet been observed in the dorsal valve. becomes evident, that the existence or non-existence of these cavities, is not always a character of generic value. Whether it be so or not, in any particular instance, depends upon the extent to which the cavities are developed. They may be so small and rudimentary, as not to be even of specific value. Or they may be so large, as to constitute good sub-generic characters. I have some specimens which seem to show that small cavities also exist in species that, with our present knowledge, can only be referred to the genus Monomerella. In a general way, therefore, it may be said that these genera are destitute of cavities, but that, exceptionally, they do occur, and that where such is the case, an approach to the genus Trimerella is indicated.

I consider that Obolellina, Monomerella and Trimerella, are merely sub genera of a single great genus, of which the first, as it is the most ancient, and the least specialized, should be regarded as the type. They gradually pass into each other, and no doubt as the number of species increases, it will become more and more difficult to draw lines between them.

The Canadian species are O. Canadensis, O. Galtensis and 0. magnifica. The second of these, has the muscular impressions in the dorsal valve of the same form and arrangement as those of the first named. The beak of the ventral valve is very large, its length being one-half that of the body of the shell. It is slightly incurved. The area has three furrows, the peduncular and the two lateral grooves. The muscular impressions are rhomboidal rather than ovate, and confined to the central portion of the shell. There are no cavities under the area.

I am informed that it is now proposed to place O. Galtensis in one of Prof. Hall's unpublished genera, presently to be noticed, along with the species described in the 20th N. Y. Reg. Rep. p. 368, under the name of Obolus Conradi. It seems to me however, that this latter is a Trimerella, or rather one of those forms whose position is near the dividing line between Trimerella and Obolel lina. Prof. Hall has figured the cast of the interior of a ventral valve in Pl. 13, fig. 2, of the work cited. Close to the area there are two short obtuse cones, which are continued towards the front, as two rounded ridges, one on each side of the muscular impressions. The latter extend nearly up to the area, and are separated by a small rounded ridge. These characters are all seen in the cast of the ventral valve of Trimerella. They do not occur at all in either of the three species of Obolellina.

Prof. Hall's fig. 1 represents the cast of the interior of the dorsal valve of his Obolus Conradi, showing that the three muscular impressions are completely concealed by two sub-conical projections, just as they are concealed by the cones in Trimerella. I have lying before me fifteen casts of the interior of O. Galtensis, and in all of them the three scars are entirely exposed as in fig. 6, below. With all due deference, therefore, I think that O. Conradi should be classified in Trimerella rather than in the same genus with O. Galtensis.

FIG. 6. The original figure of O. Galtensis showing the cast of the interior of the dorsal valve. Compare with Fig. 2. The specimen is imperfect but it shows the casts of the groove dd, the two large ovate scars, bb, and the smaller pair, c, of fig. 2.

8. Dorsal valve of Obolellina? magnifica. This was figured in the Report of the Geological Survey of Canada for 1857, published in 1858 as a dorsal valve of O. Canadensis. It is, however, a distinct species. The following is the description.

O? MAGNIFICA, n. sp. Dorsal valve transversely broad ovate; width about one-fourth greater than the length; uniformly and moderately convex; apical angle about 120 degrees; cardinal edges nearly straight, or gently convex for about one-third the length of the shell; sides and front rounded, the latter more broadly than the former. The area seems to be obsolete altogether or merely linear.

The ventral valve is depressed convex with a large beak slightly incurved. Arca with a wide triangular peduncular groove; no lateral furrows. Surfaces of both valves concentrically marked with imbricating lines of growth.

In a specimen, which appears to have been about 20 lines in length, the height of the area is nearly 3 lines.