of the external surface, more especially from the outer convex margin; I possess many examples of shells with spines, as shown by c, in Fig. 48, and in one of remarkable beauty the spines, which are prismatic, are arranged in a radiated manner: from this cause the specimen has received the name of the of the radiated Sideralite. At first sight the shell, which also exhibits the foramina very beautifully, appears as if covered with minute Diatomacea: like the preceding species, it was obtained from the Straits of Magellan.

The least troublesome plan of obtaining specimens of these foraminiferous shells is to procure from a dealer in sponge some of the sand out of the bins in which the sponges are kept; it will almost always be found to contain several species, and they can be easily picked out by sifting, or with a fine-pointed sable pencil. Some sponges have about 75 per cent of their weight made up of sand, these yield an abundance of Foraminifera.

Of all the sands I have mentioned, however, none can be compared, for richness in these shells, to that obtained from the shores of the classic island of Delos; in some specimens-as, for instance, that represented by в in Fig. 48-the last formed chambers are prolonged in a straight line like the beak of certain Ammonites from the Oxford clay. Two of the more common kinds are shown by E and F in Fig. 47; occasionally some of these are so fractured that the interior of the chambers may be well seen.

The specimens that I have hitherto described have the shell or skeleton composed of calcareous material; it not unfrequently happens, however, that this has been replaced by silica. In sections of flints, Foraminifera of various kinds are commonly met with in which the shell has been converted into silica; but in Fig. 47, C D, are shown two perfect specimens from the chalk in which the same change in the chemical composition of the shell has taken place, they can be boiled in nitric acid without injury.

A, a foraminiferous shell of the genus Clavulina; B, beaked shell from Delos; c, shell with spines, from the Straits of Magellan; D, remains of Rotalia from the chalk; E, Rotalia found in flint (Mantell).

Not only do we find in the chalk and flint the shells of Foraminifera, but even the soft parts of the animal also, as has been abundantly shown by

Mr. Deane of Clapham, and by the late Dr. Mantell. The remains most common are those of the genus Rotalia, one of which is represented by E in Fig. 48, with the body of the animal almost entire, as found in a section of flint; a portion of another is shown at D, both being copied from Dr. Mantell's paper, in the "Philosophical Transactions" for 1846.

All the Foraminifera above alluded to, have been rendered transparent by being mounted in Canada balsam; but as it is frequently a difficult matter to get rid of the air from the chambers, which, when present, completely obscures all the minute markings, I will briefly point out the method by which this can be done with comparative certainty. The shells having been covered with rather fluid Canada balsam, must be placed under the exhausted receiver of an air-pump, and allowed to remain for some time. The best apparatus for this purpose is a strong copper box, sufficiently large to hold two or three slides on its upper surface, this is to be filled with boiling water, and when the slides have been laid on the top, the box is placed under the receiver, and the air exhausted. The balsam being thus preserved in a very fluid condition, and the air contained in the chambers being at the same time expanded both by the heat, and by the removal of the atmospheric pressure, it escapes, and the balsam runs in, to occupy the

vacuum.

LECTURE VI.

SKELETON OF NUMMULITES,
ORBITOIDES AND ORBITOLITES.

FROM the Foraminifera I proceed to examine and describe the structure of a class of organized beings, of which few, if any, now exist in a living state, but which were so abundant in the earlier periods of the earth's history, as to contribute largely to the formation of some of its strata; these are the Nummulites, so named from their resemblance in form to pieces of money, or Nummi. According to Sir Charles Lyell,* the nummulitic formation, with its characteristic fossils, plays a far more conspicuous part than any other tertiary group in the solid framework of the earth's crust, whether in Europe, Asia, or Africa, it often attains a

* Manual of Elementary Geology, 3rd Edition, p. 234.

thickness of many thousand feet, and extends from the Alps to the Appeninnes. It is found in the Carpathians, and in full force in the North of Africa, as for example, in Algeria and Morocco, It has also been traced from Egypt into Aisa Minor, and across Persia by Bagdad to the mouths of the Indus. I may mention, as an interesting fact, that the great pyramid of Egypt is built of Nummulitic limestone. Strabo alludes to these nummulites under the supposition that they were lentils which had been scattered about by the workmen and had become converted into stone. One species, Nummulites complanata, is exceedingly common in the London clay formation; and Bracklesham Bay is one of the localities in which this species occurs in great abundance.

For a knowledge of the minute structure of Nummulites we are principally indebted to the labours of Dr. Carpenter, M. D'Orbigny, and Prof. Williamson of Manchester. According to these authorities, they vary in diameter from the size of a penny piece to almost microscopical dimensions, they are flattened and circular, of a more or less discoid figure; as shown by A in Fig. 49, most of them are bi-convex, as represented by D, their external surfaces are smooth like pebbles, and exhibit few, if any, markings that are visible to the naked eye. The skeletons of two other forms of organized beings have been con

* Quart. Journ. of Geol. Soc. Vol. VI, Feb. 1850.