moulding the lime in a homogeneous form, but that the crystalline force had overcome the usual mode of deposition; the absence of the cellular character in all parts of the horn being a consequence of the speedy coalescence of the cell-walls, whereby a thin layer of structureless membranous substance is produced.

The inner or nacreous layer of shell is generally supposed to be formed upon the external surface of the mantle, whilst the outer layer is developed in connection with its margin. On many shells there is an outer brown coating or epidermis of horny material; this is met with in our common Mussel, and exhibits a cellular structure, but it is more abundant, and therefore more evident, in some of the univalves, as, for instance, certain species of Triton and Conus, where it forms a layer th of an inch in thickness. This layer is termed periostracum by Conchologists, and our attention will be again directed to it when we consider the structure of those shells in which it is most abundant.

I have now described the general structure and the chief varieties of shell in the Conchifera; in the next Lecture I shall enter more minutely into the varieties peculiar to the orders into which this extensive class of animals is divided by Zoologists.

LECTURE XVII.

SKELETON OF MOLLUSCA-BRACHIOPODA.

IN the present Lecture I shall apply our general knowledge of the structure of Shell, to the two primary divisions of the Conchifera, the Brachiopoda, or Palliobranchiata and the Lamellibranchiata That the Brachiopoda-so named from two arms said to protrude from the shell-were very numerous in our seas in the early geological periods, is made evident by the fact that upwards of six hundred species have been described as British; but at the present time we can enumerate only four living representatives of the order.

One species of Terebratula has been dredged up within the last few years on the coast of Ireland. I do not possess a British specimen; all that I have are from Australia and the Navigator's Islands, and

were presented to the College by Captain Sir E. Home. They are of two species, the one a large white shell, the T. Australis, the other red, and named T. rubicunda; the former generally inhabit very deep water, being not unfrequently found at a depth of seventy or eighty fathoms; they are attached to each other, or to the rocks, by a short pedicle, which is capable of being protruded through a circular foramen in the larger valve of the shell; strong muscles are connected with this pedicle, the tendinous fibres of which are remarkable for their brilliancy. In the interior of the shell, and attached to the smaller valve, is a very beautiful framework composed of thin plates of shell substance, as shown at B, in Fig. 168, which, from its resemblance

A, Terebratula (Atrypa) psittacea showing the arms, as formerly and erroneously represented. B, smaller valve of T. Australis exhibiting the carriage-spring framework.

to a coach-spring, has been called the " coach-spring" apparatus, and the shells themselves are commonly known as Coach-spring shells. This framework seems,

at least in some species, to serve two purposes, the principal one being the support of the long spiral arms, as shown at A, the other probably, like that of the elastic ligament of the oyster, to keep open the valves of the shell. But, however beautiful this framework may be in the Terebratula, it is far surpassed by a similar apparatus in certain fossil Brachiopods of the genus Spirifer, which are very abundant in the Oolite, especially that of Ilminster in the county of Somerset. Here the framework occupies the greater portion of the shell, each spring, as it were, resembling a conical screw, and having as many as twelve spiral turns composed of flattened shelly matter fringed with minute spines along the lower margin, as shown in Fig. 169; this apparatus

FIG. 169.

A magnified view of Spirifer rostratus showing the carriage-spring framework occupying the greater portion of the shell.

no doubt performed a

similar office to that of the framework in the recent specimens. The arms, which we should suppose to be instruments of progression, are certainly not so, because the shells are firmly anchored by the short tendinous pedicle; they are clothed with

cilia, and there can be no doubt that these produce currents in the water, by which food is brought towards the animal,

The arms in the Terebratula are said to be capable of being protruded from the shell, but in the Brachiopods existing in the earlier periods of the earth's history, the arms were attached to the carriage-spring framework, and were therefore incapable of being extended; in all probability such was the case in the Spirifers before noticed. A beautiful series of dissections of Terebratula Australis by Mr. Goadby, in which the entire anatomy of these interesting animals is displayed, may be seen in the Museum of the College. The mantle is shown lining the shell, and the arms folded upon the carFIG. 170. riage-spring framework; the respiratory

and digestive systems form only a small part of the animal, being confined to the minute space within the carriage framework; the shining fibres seen in one of the specimens are those of the tendons of the pedicle.

Another existing Brachiopod I shall briefly notice is the Lingula anatina, Fig. 170. In this animal the shell is exceedingly thin, and of a green colour, whilst the pedicle is much larger than the shell itself, and not protruded through a hole in one valve of the shell, but attached to both equally. The Lingula is not found at such great depths as the Terebratula, Shell of Lin- being confined to parts nearer the surface; it with its pedicle. is generally, but erroneously, represented as being provided with an extended arm on each side, but

gula anatina