shape, of one of which the following figure furnishes an example. In this, it will be noticed, the cavity is rhomboidal, and might at

first be mistaken for the impression left by some foreign substance which had disappeared. No substance having such form, has been found attached to any of the crystals from this place, and although the angles at which the sides meet each other, (about 76° and 104°) are nearly those of calcspar, or carbonate of iron, it is evident from the enlargement of the cavity towards the surface of the crystal, or the hopper-like appearance assumed by it, that neither of these substances could have produced it. The quartz, while depositing itself around either of them, would have taken the exact form of either, precisely as we see such impress of their forms in other crystals of subsequent formation. So it is evident, that if any substance ever occupied the cavity, it must have received its form from the cavity, without communicating any to it; and thus a pseudomorphous crystal may have been produced in a manner somewhat different from usual. It is not easy to trace any relation between these last named cavities and the crystalline structure of the quartz; their sides are not parallel with any of the striæ as seen upon the faces of the crystals; they indicate an interruption in the process of crystallization, by some cause not easily explained. The cavity measures one inch on a side, and is one half

inch in depth. It is exactly represented by the two small figures below, drawn in full size from a cast of it. The crystal itself, rhombifère of Hauy, has the replacement of rhomboidal planes on the adjacent lateral

angles of the prism, and not, as is usually the case, on the alternate angles. It is permeated in every part by acicular rutile. It is possible that a comparison of other specimens, should any more be found, will result in showing an agreement between the sides of these cavities and the edges of the small rhombic replacements lettered s. In this crystal, owing perhaps to some distortion, there is not so close an approximation to parallelism between the edges of the cavity and of these replacements, as would otherwise have obtained. No such cavity as this had been described before, and if the view here taken of it be correct, we have only to suppose a successive retrocession or withdrawal of particles in such parallel directions, thus enlarging the cavity outwardly, as the crystal itself increased in size. Something of the same kind is observed in artificial saline crystals. Several smaller cavities of the same form are presented by other crystals from the same locality, in some of which, delicate needles of rutile pass through the cavity unbroken — their extremities deeply penetrating the quartz.

ON THE APPLICATION OF THE PRINCIPLES OF ACOUSTICS TO THE CONSTRUCTION OF LECTURE-ROOMS, &C. BY PROF. HENRY. [Not received.]

This communication was followed by remarks from President EVERETT and President SPARKS.

ON THE DIFFERENCES BETWEEN PROGRESSIVE, EMBRYONIC, AND PROPHETIC TYPES IN THE SUCCESSION OF ORGANIZED BEINGS THROUGH THE WHOLE RANGE OF GEOLOGICAL TIMES.

It was a great improvement in our zoological investigations when the differences in their relations, according to the various degrees of

affinity or analogy, which exist between animals were pointed out, and successively better understood. In earlier times, zoologists made no distinction between the different relations which existed among animals. Affinity and analogy, so dissimilar in their essential characters, were constantly mistaken one for the other; and upon the peculiarities which struck the observer most at first sight, animals were brought together, sometimes upon the ground of true affinity; sometimes also, upon the ground of close analogy. And, though com, parative anatomy did put the mistakes arising from such confusion right, by showing that external appearances were sometimes decep tive, and that a more intimate knowledge of internal structure was necessary fully to understand the real relations between animals, there remained, nevertheless, a degree of uncertainty in many cases, as long as the principles of affinities and of analogies were not fully distinguished. Every naturalist now knows that true relationship affinity, depends upon a unity in structure, however diversified the forms may be, under which their fundamental structure is displayed. For instance, the affinity of whales and the other mammalia, was not understood, before it was shown that, under the form of fishes, these animals had truly the same structure as the highest Vertebrata.

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Again, the forms of Cetacea exemplify the analogy there is between whales and fishes. They are related to mammalia; they are analogous to fishes. They bear close affinity to the mammals which nurse their young with milk; they have rather close analogy to the gill-breathing fishes.

Since the fossil animals which have existed during former periods, upon the surface of our globe, and which have successively peopled the ocean and the dry land, have been more carefully studied than they were at the beginning of these investigations; since they are no longer considered as mere curiosities, but as the earlier representatives of an order of things which has been gradually and successively developed throughout the history of our globe, facts have been brought to light, which now require a very careful examination, and will lead to a more complete understanding of the various relations which exist between these extinct types, and those which still continue to live in our days. Upon close comparison of these facts, I have been led to distinguish two sorts of relations between the extinct animals, and those of our days, which seem to me to have been either overlooked or not sufficiently distinguished. Indeed, the general results derived.

from Palæontological investigations, seem scarcely to have gone beyond showing that the animals of former ages are specifically and frequently, also, generically distinct from those of the present creation; and also to establish certain gradation between them, agreeing more or less with the degree of perfection which we recognize between the living animals according to their structure.

It is now pretty generally understood that fishes, which rank lowest among the Vertebrata have existed alone during the oldest periods; that the reptiles, which in the gradation of structure rank next above them, have followed at a later period, that still later the birds, which according to their anatomy, rank above reptiles, have next made their appearance and that mammalia, which stand highest, have been introduced last, and even among these, the lower families seem to have been more numerous, before the higher ones prevailed over them. Man at last has been created only after all other types had acquired their full development. These facts, which, in such generality are fully exemplified in every country in the order of succession of the different fossils characteristic of the various geological deposits, show plainly that a gradation really exists in this succession, and constitutes one of the most prominent characters of the development of the animal kingdom as a whole.

If we investigate, however, this gradation, and the order of succession of animals more closely, we cannot but be struck with the different relations which exist between the fossils and the living animals. Many extinct types have been pointed out as characteristic of different geological periods, which combine, as it were, peculiarities, which at present are found separately in different families of animals.

I may mention as such, the Ichthyosaur, with their fish-like vertebræ, their dolphin or porpoise-like general form and several special characters reminding us of their close relation to the Crocodilian reptiles; thus combining characters of different classes in the most extraordinary manner.

Again, the Pterodactyli in which reptilian characters are combined with peculiarities reminding us both of birds and bats.

Again, the large carnivorous fishes of the coal period, combining peculiarities of the Saurians, with true fish characters; and so on.

These relations are of an entirely different kind from those which I have pointed out between some of the older fossils and the early stage of growth of the living representatives of the same families. For instance, the fossil fishes with a heterocercal tail, found below the

New-Red Sandstone, down to the lowest deposits, remind us of the peculiar termination of the vertebral column in all fish-embryos of species living in the present period, to whatever family they may belong, indicating a similarity of structure in the oldest representatives of this class, with the earliest condition of the germs of those animals in our days.

Let us now examine whether we can properly understand the bearings of these relations, and the meaning of such differences.

In the first place, I have mentioned the gradual progress, which is observed in the succession of the different classes of Vertebrata. This progress is exemplified by a series of types which differ from each other, but which show, when arranged in a series, a gradation, which agrees, in general, with the structural gradation, which we may establish upon anatomical evidence. For instance, the Salamanders with their various forms rank below the tailless Batrachians.

And where we have a succession of these animals in the tertiary deposits as they occur in various parts of Europe, we may fairly say that the fossils form, in their succession a series of progressive types.

Another example may perhaps illustrate the point more fully. The Orthocera of the oldest periods precede the curved Lituites, which, in their turn are followed by the circumvolute Nautilus. Here, again, we have a natural gradation of a series of progressive types. Again, among Crinoids, we find in the older deposites a variety of species resting upon a stem, while free crinoids begin to appear only during the secondary deposit and prevail, in the present creation, over those attached to the soil. Here, again, we have a series of progressive types developed successively, which are apparently independent of each other and seem to bear no other relation to each other than that arising from the general character of the group to which they belong. Such types exemplify simply in the groups to which they belong, a real progress in the successive development of the peculiarities which characterize them as natural divisions among animals. Such forms I shall call Progressive Types.

The relations, however, which are exemplified in the oldest Sauroid fishes, in the Ichthyosaurians, in the Pterodactyls or in the Megalosaurians, seem to me to be clearly of a different character, and to differ from simple progressive types, inasmuch as those which appear earlier, combine peculiarities which at a later period appear separately in distinct forms. For instance, the reptilian characters which we recognize in the Sauroid Fishes, are developed at a later period in