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that very few spots have been observed outside of the zones lying 40° on either side of the equator. I know of but two instances on record in which spots have been observed beyond this limit. La Hire observed a spot 70° from the equator, and more recently, in the month of June, 1846, Dr. C. H. F. Peters observed at Naples a spot 50° from the equator.

It is further to be remarked that according to the conclusions of the English observers, the solar spots attain higher latitudes during the years of the maximum number of spots, and recede more and more towards the equator as the minimun is approaching; and it is to be noted that the present year is precisely, or at least very nearly, a minimum year. It is doubtless owing to the unusual thinness of the chromosphere during this period that spots have been observed in so high latitudes this year. It is true that the spots were small, but, nevertheless, they were genuine spots, with all the characteristics of larger spots.

It is difficult for one who has seen the phenomena which I have described, to come to any other conclusion than this: that the veiled spots are breaks or true openings in the photosphere, seen through the imperfectly transparent gases composing the chromosphere, openings themselves partly or wholly filled by the vapors ejected by the forces from the interior of the photosphere. If this bypothesis should prove to be the expression of a fact, then we should expect to find that the photosphere is perforated by thousands of crevasses either partly or entirely filled with the vapors and gases from the interior, which cannot be ejected outside for want of sufficient energy, save for a comparatively very small number situated in the equatorial zones, where this energy appears maximum, and is able to repel and dissolve the gases from the interior.

Before the observations of this year, I had arrived at precisely the same conclusions in regard to the opening of the photosphere in all latitudes, and to the existence of invisible spots concealed by the chromosphere. These conclusions were derived from my observations with the spectroscope, made at Harvard College observatory during a period of thirty-five months. A discussion of these observations is reserved for a future communication.

Though one can hardly form a settled opinion with regard to the cause of the general depression of the chromosphere, on account of the imperfect data, it seems natural, however, to suppose that the phenomenon is connected in some way with the minimum period of sun spots. Judging by the great number of veiled spots observed, and by the myriads of pores seen between the granulations, it would seem that both the chromosphere and photosphere have been much thinner than usual during the present year.

If there are breaks in the photosphere at many points of the surface of the sun, it becomes easy to account for the unusual thinness of the chromosphere this year, because as observed by myself and others, at certain phases of the spots, the chromospheric gases, rushing with impetuosity into the umbra, go down under the photosphere like gigantic waterfalls, diminish. ing consequently the thickness of the chromosphere. That this takes place I shall give ample proof in another communication.

It seems evident that the chromosphere near a spot is kept from falling into the opening by a force from the interior. As soon as this force decreases in energy, immediately the chromosphere tends to cover it, and even to precipitate itself through the opening when this force becomes extinct. The observations show this plainly.

When a spot is decreasing, it is quite common to observe that the umbra and penumbra appear as if they were seen through a heavy fall of snow, their surfaces being covered by numerous bright flocculent granulations surrounded by a kind of bluish fog. In a few instances of very rare definition, I have been surprised to see faint traces of this flocculent appearance upon almost all the spots ; indeed it would seem that the spots are rarely free from some faint traces of the chromospheric gases. Probably the bright flocculent objects observed upon the umbra and penumbra of spots, are the granulations of the chromosphere dissolved to a greater or less degree by the forces emanating from the spots.

Perhaps it may not be idle to remark that, during the period mentioned, I have almost every day observed small groups of faculæ in the polar regions, especially near the north pole of the sun; while, for the most part, they have been entirely absent from the equatorial regions, where they are commonly found.

To conclude, my observations show:

1. That during this year, and especially during the interval from June 10 to August 18, and to a less degree to September 14, the chromosphere has been notably thinner than usual upon the entire surface of the sun.

2. That the granulations have been smaller and less nu. merous.

3. That the light-gray colored background seen between the granules has been more conspicuous and has occupied more space than usual.

4. That there are spots, which I have named “veiled spots," which are seen through the chromosphere which is spread over them like a veil.

5. That these veiled spots are true openings of the photosphere, like those of the ordinary spots.

6. That during this period these spots have been larger, darker, and more numerous than I have before seen them.

7. That the veiled spots are scattered throughout all latitudes, though more complicated in the regions wbere the ordinary spots make their appearance.

8. That I have observed spots at least within 10° of the north pole of the sun.

9. That the flocculent objects sometimes seen projected upon the umbra and penumbra of spots are the remaining portion of the granulations composing the chromosphere, more or less dissolved by the forces emanating from the interior of the photosphere.

Cambridge, October 1, 1875.

ART. XX.- On the structure of Obolella chromatica ; by E.

BILLINGS, F.G.S.

THE genus Obolella was founded in 1861,* on the following three species of Brachiopoda:

1. O. chromatica, discovered by J. Richardson in 1861, at a place called “L'Anse au Loup," on the north shore of the Straits of Belle Isle, in Labrador.

2. 0. crassa Hall, from Troy, in the State of New York. 3. O. polita Hall, from Wisconsin.

Figg. 1, 2.–Ventral valves. The beak is not seen in either of the specimens.

Fig. 3.-Diagram showing the position of the scars in the dorsal valves. All these figures are enlarged about 24 diameters.

The specimens exhibited the internal characters very imperfectly, yet enough was seen to convince me that the genus was a new one. During the fourteen years that have elapsed, I have received a number of letters, from both American and European authors, inquiring for more complete details of the structure of 0. chromatica, which has always been considered to be the type. This information I was unable to give, for want of the facts. We are now in possession of specimens showing the interiors of both valves, almost completely. The following are the characters as nearly as they can be made out:

In the ventral valve there is a groove in the hinge line, for the passage of the pedicel. On each side of the groove there is a small, somewhat deeply excavated cardinal scar. In the cavity of the valve there are two elongated scars, which extend

* Geology of Canada, Palæozoic Fossils, vol, i, p. 7, 1861.

there sport be uppeforwandt probo find

from near the cardinal scars forward about two-thirds of the length of the shell. These diverge from each other, more or less, in their extension forward, and are usually curved but sometimes nearly straight. They may be called laterals. They are, in general, separated from each other about one-third of the width of the shell. A little above the mid-length, and between the two laterals, there is a pair of small scars arranged transversely, with their inner extremities directed somewhat forward. The space above these two scars, between the upper portion of the laterals, is generally tumid from the thickening of the shell. In one of the specimens there is a small pit in the center of this space.

The dorsal valve has a small area, or nearly flat hinge facet. The minute beak is slightly incurved over the edge of the area. Beneath the beak there is a small sub-angular ridge, on each side of which there is a cardinal ? scar. The elongated scars, which seem to correspond to the laterals of the ventral valve, are here altogether in the upper half of the shell. They diverge widely in their extension forward. They are in general very slightly impressed, and would, most probably, escape the observation of any one who did not expect to find scars where they are situated. In the cavity of the valve there is a low rounded median ridge, which extends from a point near the hinge line forward a little below the mid-length of the valve. About the middle of the shell there are two small scars. These are usually striated longitudinally. The median ridge passes between them. The area is coarsely striated.

The above are the principal characters of this species, and they are subject to some variations, one of which is particularly worthy of notice. The two small cardinal scars of the dorsal valve are sometimes elongated laterally. This is carried to such an extent in another species (0. gemma) that they not only extend the whole length of the hinge-line, but are curved forward at their outer extremities and continued down into the cavity of the valve. In such cases they present an appearance similar to that of the groove beneath the hinge-line of the genus Obolellina. In other species of this genus the lateral scars of the dorsal valve are sometimes connected together by their upper extremities. But this is not a constant character. In different individuals, of the same species, these scars are either connected or not. The laterals are also sometimes connected with the cardinals.

The following are the original figures published in the Paleozoic Fossils, p. 7, (1861):

Fig. 4, Q, Ventral valve; b, dorsal; c, interior of ventral valve, showing the muscular impression; d, outline on a side view, restored from detached valves. Natural size.

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ventral valve, showing

Fig. 4

In the description it is said: “Muscular impressions in the ventral valve, four; one pair in front of the beak, near the middle or in the upper half of the shell.” The pair bere alluded to are the laterals. Their upper and lower extremities are sometimes not visible, and what remains occupies the middle portion of the length of the shell. The expression “or in the upper half," I can thus explain : I had the dorsal valve of O. crassa, from Troy, which I then supposed to be a ventral valve. In this the laterals are in the "upper half." The transverse scars were not then observed and hence four scars instead of six. It must be borne in mind that fourteen years ago nothing was known of the internal characters of these shells. The materials were imperfect and consequently so was the description. It is now certain that the genus is a good one and that all of the three species on wbich it was founded belonged to it.

The described species which I consider to be truly within the genus are: 0. chromatica, 0. polita, O. crassa, 0, nuna, and 0. gemma. They all, so far as is yet known, are confined to the Potsdam Epoch. A number of other species have been referred to the genus, but they are all more or less doubtful.

The specimens which have furnished the above additional details of the structure of 0. chromatica were collected at L'Anse au Loup, the only place where the species has been found, in 1863, by T. C. Weston of our Survey, and by him very skilfully worked out of the matrix.

Art. XXI. - On the Damming of Streams by drift ice during the

melting of the great Glacier ; by J. D. DANA. WHEN treating of the overflows of the flooded Connecticut, in the Supplementary December Number of this Journal, (p. 497,) I suggested, in view of the fact that the terraces in the Farmington Valley about Tarifville and Simsbury are at least 50 feet higher than those a mile eastward in the parallel Connecticut valley—that the gorge through the Divide Range, by which the Farmington river there passes into the Connecticut valley, had been closed by drift and so remained until the flood had reached its height.

I allude to this subject again to add that the events connected with the opening, in the Spring, of many of our modern icecovered streams afford abundant reason for believing that, during the breaking up of the long Glacial winter, when the melting was going forward, the gaps, gorges or narrows, along the river courses, would have been liable to obstruction by floating ice.

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