Coloured Pearls.                          261
	fancies, however, we may refer to the opinion so often expressed and still entertained in some quarters, that the black colour of a Pearl is traceable to some disease in the Pearl-bearing mollusc. Although the origin of the colour is in the deepest degree obscure, it seems probable that it is in some cases due to the presence of certain pig­ments in the medium in which the molluscs live. The subject of the colouring matter of the nacre in the shells of the genus Unto, afforded matter for an interesting discussion at the meeting of the Academy of Natural Sciences of Philadelphia, on March 20, i860. If we know the nature of the pigment which colours the nacreous lining of the shell, we may safely conclude that we know also the character of the colouring matter in the tinted Pearl ; inasjnuch as a Pearl is of precisely the same nature as the nacre of its shell Here we refer not to the pearly hue of a nacreous shell, which, as ex­plained in an early chapter (p. 87), is a purely optical phenomenon, but to the substantive colour of the carbonate of lime which constitutes both the nacre and the Pearl, and which colour is, no doubt, due to the presence of some material pig­ment. The late Dr. James Lewis, of Mohawk, New York, suggested that the colour of many fresh­water shells might be caused by certain salts of