CONCiriFEllA. 



713 



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- 

 pression 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 arid 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. 



Various causes have been assigned to explain 

 the formation of pearls. But it seems enough 

 to be aware in u general way of the manner in 

 which bivalve shells grow to understand how 

 pearls 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 ofAnnelidan, 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 laminae 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 arc 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- 



VOL. I. 



come free in the substance of this muscle, and 

 there continue to increase \vith 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 addm t..> 

 muscles of a conchiferous animal. 



Fig. 369. 



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 papillae 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 pa >.l 'leal 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 



3 A 



