Outside of the shell. Inside of the shell. ] [Illustration: Fig. 165. Meleagrina margaritifera (Linnæus).] But the most interesting of all the nacre-bearing shells is the pearl oyster (_Meleagrina margaritifera_), the exterior, as well as the interior, of which is represented in Fig. 164. The interior of the shell affords the most exquisite pearls; the Esterhazy collection of jewels afforded many such specimens. This shell is nearly round, and greenish in colour on the outside; it furnishes at once the finest pearls, under favourable circumstances, and the nacre so useful in many industrial arts. Fine pearls and nacre have, in short, the same origin. The nacre invests the whole interior of the shell of _Meleagrina margaritifera_; being the same secretion which in the pearl has assumed the globular form: in one state it is deposited as nacre on the walls of the bivalve, in the other as a pearl in the fleshy interior of the animal. This nacre is therefore at once a calcareous and horny matter, which the animal secretes, and which it attaches to the interior walls of the shell during the several periods of its development. Pearls are formed of the same substance, only in place of being deposited upon the valves in beds, the material is condensed and agglomerated in small spheroids, which develop themselves either on the surface of the valves or in the fleshy part of the mollusc. Between nacre and pearls, therefore, there is only the difference of the form of deposition. Fig. 165 represents the pearl oyster with calcareous concretions in various states of progress. The finest pearls--solidified drops of dew, as the Orientals term them in the language of poetry--are secretions supposed to be the result of disease in the animal. The matter, in place of being spread over the surface of the valves in their beds, is condensed either on the centre of the valves or in the interior of the organ, and forms a more or less rounded body. The pearls, when deposited on the valves, are generally adherent; those which originate in the body of the animal are always free. Generally we find some small foreign body in their centre which has served as a nucleus to the concretion, the body being perhaps a sterile egg of the mollusc, the egg of a fish, a rounded animalcule, a grain of sand even, round which has been deposited in concentric layers the beautiful and much-prized gem.