perfectly free from carbonate of lime, as to render it highly probable that in other specimens, having the same angle, but containing also carbonate of lime or other ingredients intermixed, the form is really dependent on the carbonate of iron! alone.

It appears, however, not unlikely that when substances, which agree so nearly in their primitive angle, are intermixed in certain proportions, they may each exert their power; and may occasion that confused appearance of crystallization with curved surfaces, known by the name of pearl-spar. I cannot say that I have made any accurate comparative analyses which may be adduced in support of the hypothesis, that mixtures are more subject to curvature than pure chemical compounds; but it is very evident, from the numerous analyses that have been made of iron-spar by other chemists, how extremely variable they are in their composition, and consequently how probable it is, that the greater part of them are to be regarded as mixtures; although it be also possible, that there may exist a triple carbonate of lime and iron as a strict chemical compound.

It seems not unlikely, that there may hereafter be found some carbonate allied to the preceding, which may owe its form to the presence of manganese; but notwithstanding the liberality which happily prevails in general among those who have it in their power to assist in such inquiries, I have not had the good fortune to meet with any such compound; and I am unwilling, merely in the hope of making such an addition, any longer to defer communicating an observation, which I hope will be of real utility in the discrimination of bodies that differ so essentially in their composition.

IX. Observations intended to show that the progressive Motion of Snakes is partly performed by means of the Ribs. By Everard Home, Esq. F. R. S.

ON

Read February 27, 1812.

N a former occasion I laid before the Society a description of the mechanism of the hood of the cobra de capello snake of the East Indies, the coluber naja of Linnæus, in which the ribs of the neck are shewn to be formed in a particular manner; so that when they are raised, the skin becomes stretched out, and puts on the appearance of a hood.

The ribs so employed have several peculiarities, which, I took for granted, were confined to those of the neck, for I was not in possession of the bodies of the snakes, and therefore could not examine the others; but have since found that many of these peculiarities are not only common to all the ribs of this snake, but to those of the whole tribe.

This fact, as it escaped my observation at that time, would have still done so, had it not been for the following circum

stances.

A coluber of unusual size, lately brought to London to be exhibited, was shewn to Sir JOSEPH BANKS; the animal was lively, and moved along the carpet briskly: while it was doing Sir JOSEPH thought he saw the ribs come forward in succession like the feet of a caterpillar. This remark he immediately communicated to me, and gave me an opportunity of seeing the snake and making my own observations.

So,

The fact was readily established, and I could feel the ribs with my fingers as they were brought forward; when a hand was laid flat under the snake, the ends of the ribs were distinctly felt upon the palm, as the anitnal passed over it.

This becomes a more interesting discovery, as it constitutes a new species of progressive motion, and one widely different from those already known.

In the draco volans the ribs form the skeleton of the wings, by means of which the animal flies, the five posterior ribs being bent backwards and elongated for that purpose, so that in that instance the progressive motion is performed by the ribs, but those particular ribs are superadded for this purpose, and make no part of the organs of respiration; whereas in the snake the ribs are so constructed, as to perform their office with respect to the lungs as well as progressive motion.

That ribs are not essential to the breathing of all animals, whose lungs are situated in the same manner as in snakes, is proved by the syren having no ribs; but as this animal has also gills, and can Breathe in water as well as in air, the lungs are not so constantly employed, and probably a less perfect supply of air to them may suffice.

In animals in general, the ribs are articulated to the back bone by means of a convex surface, which moves upon a slightly concave one formed upon two of the vertebræ, partly on the one and partly on the other, so that there is a rib situated between every two vertebræ of the back; but in the snake tribe, the head of the rib has two slightly concave surfaces which move upon a convex protuberance belonging to each vertebra, so th a there is a rib to each of the vertebræ.

One advantage of this peculiarity is, that it prevents the ribs from interfering with the motion of the vertebræ on one

another. The vertebræ are articulated together by ball and socket joints (the ball being formed upon the lower end and the socket on the upper one), and have therefore much more extensive motion than in other animals.

The muscles, which bring the ribs forward, consist of five sets, one from the transverse process of each vertebra to the rib immediately behind it, which rib is attached to the next vertebra. The next set goes from the rib a little way from the spine just beyond where the former terminates, it passes over two ribs, sending a slip to each, and is inserted into the third: there is a slip also connecting it with the next muscle in succession. Under this is the third set, which arises from the posterior side of each rib, passes over two ribs, sending a lateral slip to the next muscle, and is inserted into the third rib behind it.

The fourth set passes from one rib over the next, and is inserted into the second rib.

The fifth set goes from rib to rib.

On the inside of the chest there is a strong set of muscles attached to the anterior surface of the vertebræ, and passing obliquely forwards over four ribs to be inserted into the fifth rib, nearly at the middle part between the two extremities.

From this part of each rib a strong flat muscle comes forward on each side over the viscera, forming the abdominal muscles, and uniting in a beautiful middle tendon, so that the lower half of each rib, which is beyond the origin of this muscle, and which is only laterally connected to it by loose. cellular membrane, is external to the belly of the animal for the purpose of progressive motion; and that half of each rib next the spine, as far as the lungs extend, is employed in respiration.

At the termination of each rib is a small cartilage in shape corresponding to the rib, only tapering to the point. Those of the opposite ribs have no connection, and when the ribs are drawn outwards by the muscles, are separated to some distance, and rest through their whole length on the inner surface of the abdominal scuta, to which they are connected by a set of short muscles: they have also a connection with those of the neighbouring ribs by a set of short straight muscles.

These observations apply to snakes in general; but they have been particularly examined in a boa constrictor, three feet nine inches long, preserved in the Hunterian Museum. In all snakes, the ribs are continued to the anus, while the lungs, seldom occupy more than one-half of the extent of the cavity covered by the ribs. These lower ribs can only be employed for the purpose of progressive motion, and therefore correspond in that respect with the ribs in the draco volans superadded to form the wings.

The parts of which a description has been attempted, will be better understood by an inspection of Plates IV. and V. than by any explanation that words can convey.

In Plate VI. the joints between the vertebræ and ribs are represented of the natural size from the skeleton of a large boa, sent from the East Indies by the late Sir WILLIAM JONES and deposited in the Hunterian Museum. On the under surface of the vertebra is a protuberance for the attachment of muscles peculiar to this genus, it varies in size in the different species, and explains the power attributed to the boa constrictor.

When the snake is going to put itself in motion, the ribs of the opposite sides are drawn apart from each other, and the small cartilages at the ends of them are bent upon the upper surfaces of the abdominal scuta, upon which the ends of the