In a very considerable number of Mono myarians the lunula and corslet are replaced by certain projecting parts to which the name of auriculec or auricles has been given. These occur more especially in the Pecten family— the Pectinites of Lamarck ; they are distin guished into anterior and posterior, and they are frequently unequal.
If we now turn to the particulars of the external surface of the shell of the conchi fera, we shall find many points worthy of being attentively noted. In" a very great number this surface is covered with a thin and frequently deciduous lamina of a sub corneous and often filamentous substance, to which the title of epidermis has been given. This matter is secreted by the most external edge of the mantle, but observers have not yet stated in what manner the secretion takes place, and what means the creature employs to make this epidermis adhere so strongly to its shell. The epiderniis often occurs both of considerable thickness and extent (Glycimeris, Solemya), and thus constitutes an important portion of the shell. In other genera the epi dermis appears to be wanting entirely, and in others bears some resemblance to velvet of thicker or thinner pile, and then consists of a large quantity of short hair, standing erect, and more or less closely set. In some species these hairs become more scanty, but increase greatly in length, as we perceive in certain Archidm and Bucarides. When it occurs in certain species the successive growths of which are manifested by irregular ridges the epider mis is irregularly squamous. Tile epidermis is insufficient to furnish any generic character that can be depended on; for there are certain extremely natural genera in which some species are covered with it whilst others are entirely naked.
The other particulars of the external surface of the shell are soon glanced at : they consist of stria, ridges or ribs, and furrows, which, according to their direction, are distinguished into longitudinal when from the hook they run towards the inferior margin, and transverse when they foliow an opposite course, that is to say, when they run from before backwards ; they are oblique, again, when they follow a line in any way inclined to the longitudinal or the transverse. These stria, ridges, and furrows, may cross one another and the shell is then trellised. They may a'lso severally present a great variety of particular appearances, the de finitions of which may be found in the ordinary elementary works on Conchology, but which may all be learned much more rapidly from even a. very moderately attentive study of the shells themselves than from any written description, however minute and accurate.
Internal sudaee.—The inner surface of bi valve shells is commonly smooth and polished, and often presents different colours which de pend on the secretion of that part of the man tle which produces the solid laminm of the inner surface. The greater number of shells are white within, and many of them are na creous or like mother-of-pearl. Mother-of pearl would appear to be the consequence of a molecular arrangement of the calcareous ma.tter intimately united in a constant ratio with the animal matter by the combination of which the shell is formed. The pro portion of the two substances does not ap pear to be the same in the non-nacreous and the nacreous shells ; there are some which afford a much larger proportion of calcareous, and others which yield a much larger propor tion of animal matter when analysed than is usual. Naturalists are now generally aware of the experiments, an account of which is to be found in the Philosophical Transactions, from which it appears that the nacreous lustre is owing to the decomposition of light by an infinity of asperities of excessive minuteness which beset the surface of the shell. It has,
indeed, been found possible by means of an impression from a mother-of-pearl surface taken in sealing-wax especially, to transfer the power of exhibiting corresponding phenomena to the surface of the wax.'t' There is a variety of characters exhibited by the interior of the valves which it is of con sequence to be familiar with. In shells which have belonged to dimyary mollusks, two muscular impressions of variable depth are constantly to be found in the interior. Some times they are so superficial that they escape an examination vvhich might even be charac terized as minute. One of these impressions is on the anterior side of the shell, the other on the posterior. They are generally sub rotund ; sometimes, however, they are elon gated, which serves as an announcement that the muscles were flattened. In some genera these muscular impressions are of a particular form, as may be observed in the Lucina for example. It is a circumstance worthy of ob servation that the muscles of the animal shift their place and come forward in the shell in proportion as it grows, and it might have been concluded, a priori, that this could not be otherwise, when the mode of increment pecu liar to the class is taken into consideration. On escaping from the ovum, a conchiferous rnollusk is already provided with its shell, of course of very small size, and its two adductor muscles; and the relations of these muscles to the shell and the other internal organs are the same as at every subsequent period. When the animal has attained to some lines in length, and by the lapse of time to much larger di mensions, did not the muscles undergo a gradual displacement the shell would be found as thin at the summit as it was on escaping from the egg, and the muscles prolonged into the interior of the hook. Now, not only does the shell go on increasing in thickness and the hooks fill up, but observation shows that the adductor muscles always preserve the same relations and the same proportions. To study in the best possible manner the successive dis placements of the muscular impressions, the best mode is to saw a fossil oyster-shell length wise in a line passing from the summit through the centre of the muscular impression. The impression will then be seen beginning towards the summit and increasing gradually in its dimensions, so as to form a long triangular imprint, running obliquely through the thick ness of the shell. When, at a very early age, the shell was extremely thin, the muscular im pres8ion existed very near to the external sur face ; but in proportion as the animal has become older, and new layers of calcareous matter have been successively added to the former, the muscular impression is found to have become farther and farther removed from the external surface. It is generally on the surface of the muscular impressions, and in the substance of the adductor muscles them selves, that those peculiar solid and highly prized excrescences called pearls are produced. These excrescences are engendered in a very considerable number of genera, and it is to be presumed that they may occasionally exist in all ; it is, however, among the Monomyaria that pearls are most constantly formed.