Mr. Everett read a short paper on the number of primary planets belonging to our system which might be supposed to remain as yet undiscovered.

He stated that he had been "led more particularly to the inquiry by the striking remark of M. Leverrier (in the Compte Rendu for 5th October, 1846, p. 659) to this effect, that we may hope that, after thirty or forty years of observations of the new planet, we shall be able to use it, in its turn, for the discovery of that which follows it, in the order of distances from the sun. And so on. Unhappily we shall soon fall upon stars, invisible in consequence of their immense distance from the sun, but whose orbits eventually, in the lapse of ages, will be traced with great exactness by means of the theory of secular inequalities.' All calculations of this kind must, of course, take for granted that the law of gravitation exists and operates in the remote parts of the system as it does within the reach of our observations. The star 61 Cygni is usually regarded as the fixed star nearest to us, and this is placed by Bessel at the distance of 62,481,500,000,000 miles. Adopting the only supposition that we are able to make on this subject, viz., that our sun and the stars are bodies of equal size and density, we may conclude that the attraction of the sun extends over half the interval between the sun and 61 Cygni, that is, over a space of 31,240,750,000,000 miles. The law of the distances from the sun at which the planets succeed each other in our system is not known; but assuming Bode's law, or, still more simply, a geometrical progression, as that which comes near the truth in reference to all the known planets except the last discovered (regarding the asteroids as one planetary system), and we should have room for ten new planets, and nearly the eleventh, outside the orbit of Neptune."

Mr. Everett also read a letter from Professor Owen, of London, containing an approving notice of Dr. Meigs's paper on the generation of the opossum, and expressing a strong desire to receive, in behalf of the Hunterian Museum, specimens of the impregnated uterus of this animal, preserved in spirits.

"Female opossums killed between the 18th of February and the 6th of March would be likely to afford such specimens, — of which not one exists in the museums of London, nor, I believe, in Europe. The value of the specimen would be enhanced, if any of your young anat

omists would, previously to immersing the animal in spirits, inject the abdominal aorta with size or gelatine, colored by vermilion.

"There is only one expression in Dr. Meigs's memoir in which I am not disposed to acquiesce, where he says 'it is not to be believed that a breathing, &c., mammifer can be developed independently of a placenta' (p. 329). I have met with so many unexpected exceptions to assumed general rules in the animal economy, so many proofs that the Creator operates by ways diversified infinitely beyond human calculation, as to adopt no scientific dogma whilst any means of testing it by observation remain untried. It would be most desirable that the female opossum should be sought for at the period shown by Dr. Meigs to be that when she would be most likely to have embryos in the uterus, and the nature of the connection between the mother and offspring be examined. A placenta may be defined as a vascular, villous, or cellular process from the outer surface of the chorion, interlacing with a similar process from the inner surface of the womb, and producing an adhesion of the chorion thereto difficult to be overcome, and often not without laceration. The presence of such an organ, simple or subdivided, would be easily determined in an opossum killed towards the latter period of her brief pregnancy, say from March 1st to 7th. The analogy between the condition of the new-born young in the kangaroo, in which no placenta is formed, and the opossum is so close, that, if I were to allow myself to anticipate what unbiased observation ought to decide, I should expect as close an analogy in the condition of the fœtal membranes and appendages."

Mr. Teschemacher exhibited some specimens of anthracite coal, containing what he supposed to be fossil seeds, as he had carefully decarbonized the internal substance of them, and by the assistance of the Oberhausser microscope belonging to Professor Agassiz, with a power of 700 diameters, had found it to be a mass of distinct cells. Some of these seeds were surrounded by impressions resembling spinous processes, which were uniform and symmetrical. He also stated, that lately discovered specimens had confirmed him in his opinion, that most of the appearances usually called slickensides in the anthracite coal were the external parts of large fossil plants; also, that the small, uniform striæ frequent in the coal, and which are sometimes found covering pieces of a conical form, are the

impressions of the vessels of vegetables, and were not caused by the action of sliding in the veins; he thought that no accidental sliding could produce such uniform marks on numerous specimens from various mines.

Mr. Teschemacher also exhibited some rhizomorphæ, found in the old chambers of coal-mines.

Professor Agassiz called the attention of the Academy to the importance of a complete investigation of the anatomy, the mode of life, and the embryology of the blind-fish of the Mammoth Cave (Amblyopsis spelaus). He thought there was an opportunity to settle, by actual experiment, the extent of physical influences in causing organized beings to assume their peculiar and distinctive characteristics in relation to the media in which they live. He doubted not that important inferences could be derived from these investigations with reference to the question, whether species have been created in the localities they inhabit and with special adaptation to the external circumstances in which they were first introduced, or whether these circumstances were acting as modifying influences upon fewer primitive types, which would thus be diversified and produce the successive changes evinced in the geological series. The experiments he would suggest are, first, an accurate and complete anatomical investigation, sufficiently minute to enable the observer to perceive the slightest changes which would occur; secondly, observations upon individuals of different ages brought to the light and kept for a long time, even through a series of successive generations, in these new circumstances; thirdly, a complete series of comparative embryonic investigations with recently laid ova, traced simultaneously in the dark and under the influence of moderate and of intense light, keeping especially in view the formation of the nervous system, and particularly that of the eyes. If there is an eye formed in the dark, ascertain when and how it disappears, as it is entirely wanting in the full-grown individuals, and again notice the differences in this respect between specimens growing under the influence of light.