Various causes have been assigned to explain the formation of pearls. But it seems enough to be aware in a general way of the manner in which bivalve shells grow to understand how pear!s are produced. Their production, it would appear, may be assigned to some accident hap pening to the animal ; sometimes a few grains of sand. getting between the mantle and the shell prove nuclei for their formation, but still more frequently they are consequences of per forations made by a species of Annelidan, to the attacks of which bivalve-shelled animals are obnoxious. In either case the animal, feel ing itself injured, deposits over the grain of sand or the small orifice made by the Annel idan, a thin layer of nacreous matter, secreted accidentally and superabundantly with re ference to its regular laminva of progressive growth. In consequence of this, the shell at the point where the grain of sand lodges or where it is wounded acquires more than its usual thickness. This thickening, from the mere fact of its presence, becomes a perma nent cause of excitement to the mantle of the animal, so that this organ goes on secreting an unusual quantity of calcareous matter, in con sequence of which there results an elevation that increases with the age of the animal, so much the more rapidly as the annoyance has been greater and more permanent. When the mass has increased so much as to penetrate somewhat deeply into the substance of the organs, it is then apt to go on increasing by depositions of nacreous matter upon one of its extremities, by which we have pedunculated and elongated pearls produced. Zoologists have also asked how those pearls that are found perfectly free in the interior of conchi ferous mollusks were formed. We shall first observe that these pearls are met with more especially in the substance of the adductor muscles ; now if it be remembered that these muscles shift their place in proportion as the animal grows, it may readily enough be al lowed that a pediculated pearl developed on the surface of the muscular impression itself, might be detached from its connexion with the shell by the advance of the muscle, be come free in the substance of this muscle, and there continue to increase w ith more or less rapidity. This explanation, which we advance for the first time, appears to us sufficiently plausible ; but, before admitting it as an esta blished fact, it would be well to institute some experiments in regard to the successive changes of position undergone by the adductor muscles of a conchiferous animal.
The mantle, as we have seen, is attached to the shell by a determinate portion of its sur face. In the Dimyaria the part that is ad herent is not far from the thickened edge of the mantle ; it adheres by means of the small muscles which regulate its contractions, as well as those of the tentaculary papillze with which it is commonly fringed. In the. Monomyaria the adhesion of the mantle is situated much higher, and very nearly at the place where the lobes of the mantle are detached from the general mass of the body. From the adhe sion of the mantle to the shell there results a linear impression, to which M. de Blainville has given the name of palleal impression ; in the Dimyaria it extends from before backwards, from the anterior to the posterior adductor muscular impression, following the circum ference of the edge. This linear impression is simple when it presents no inflexion in its course. In a considerable number of the Di myaria it is observed to form a notch of dif ferent depths in different species, directed towards the mantle. This notch appears to be produced, as we have already said, by the pro per retractor muscle of the siphons.
Besides the muscular impressions of which , we have now spoken, several others of much less importance have been particularized in the greater number of the conchiferous mollusks. All the species that have a foot have peculiar muscles to move this organ, and these have their fixed point of action on some point of the interior of the shell. They are 'generally
divided into two principal fasciculi ; the one runs to be inserted within the hooks, the other in the Dimyaria proceeds to be attached before and above the posterior adductor muscular impression. In the IVIonomyaria, the foot of which is generally rudimentary and without use, we observe nothing more on each side of the body than a single small fibrous fas ciculus the impression of which is found on the inside of the hooks. In some genera of Dimyaria, and particularly in the Unio, we observe three and sometimes four muscular im pressions belonging obviously to the adductor muscles of the valves, which are occasioned by the anterior adductor muscle in particular being divided into two fasciculi, often of un equal size, as in certain Uniones, and some times equal and of considerable magnitude, as among the Iridines.
From the' summary and concise view we have taken of the principal facts in the organi zation of the Conchifera, very important con clusions may be .drawn with reference to the classification of these animals.
Taking the Conchifera, properly so called, and looking narrowly into that which is of most importance in their organization—the ner vous system, we find two principal modifica tions, coinciding in a very remarkable manner with the number of the muscles. This num ber of the muscles, permanently proclaimed by the impressions they leave on the shell, presents an important character by means of which, while we define their limits somewhat more strictly, we feel authorized in retaining the two grand orders of Lamarck,—the Conchi fera Dimyaria, and' the Conchifera Mono myaria. A fact of some importance and brought to light by the observations of? Poli, is that a small nervous ganglion exists at the point of commissure in those acephalous mol lusks which have the lobes of the mantle con joined. This peculiarity gives new conse quence to the characters drawn from the con joined or disunited state of the lobes of the mantle. Unfortunately the circumstance is not always indicated upon the shell; it is, in fact, only obvious upon those inhabited by siphoniferous animals; it is quite inapprecia ble upon those the inhabitants of which have siphons so short as not to require a particular retractor muscle to draw them within cover of the shell. With regard to the other organic characters which furnish data available in clas sifying the Conchiferous mollusks, these are all of so little permanency that they are only useful in supplying secondary hints for the arrangement of families and genera. Thus neither the branchim nor the heart present any character susceptible of generalization or of contrast. Better data might perhaps be ob tained from the conforrnation of the organs of digestion ; but these organs have hitherto been examined in comparatively so small a number of genera and species that they cannot be brought forward usefully in supplying cha racters for a general classification. If, as we ourselves feel inclined to do, the hinge be taken as the point of starting in the Pholades, this part may be made the means of giving excellent characters in its principal modifica tions for the establishment of genera. It is, indeed, very remarkable that we should find the characters as indicated by the hinge almost constantly in harmony with those af forded by the rest of the organization ; and with a few exceptions, relative to several ex tremely natural families, that of the Unios for example, all that is valuable in the generic cha racters generally may be preserved along with the characters supplied by the hinge. Ano ther character which may be usefully employed in classification is assumed from the regularity or irregularity of the shell of the animal ; in generalizing upon this, like groups are obtained in the two principal divisions of the Conchifera, and the two principal divisions of the classi fication are referred to the simplicity or exact ness of the dichotomy, whilst natural groups are preserved as much as may be in the linear arrangement.