1 72 Conchology. 



the posterior part furnished only a fluid of a different color, and 

 usually less deep than the first. By means of this principle it is 

 not difficult to account for the arrangement of the different colors 

 which are so splendidly exhibited among this class of natural ob- 

 jects. These colors may be reduced to one or more, which are 

 more vivid on a lighter ground; to colored, circular bands on a 

 ground of a less vivid color, or pure white; to longitudinal lines, 

 round or square spots, and in a regular, or irregular zigzag form. 

 All these may be easily explained, according to the principles 

 which have been laid down, the application of which, from what 

 has been said, will not, we hope, be found diflicult. 



But from this mode, which is the most general in the produc- 

 tion of the colors of shells, there are certain deviations. In that 

 division of shells which is made by some naturalists, and which is 

 distinguished by the name of porcelain shells, on account of the 

 fine enamel with which they are covered; there are two sets of 

 colors, which are disposed in a parallel direction to each other. 

 The external range of these colors is owing to a peculiarity of 

 structure in the animals which inhabit them, different from that 

 of other testaceous animals, and to an operation which does not 

 take place in other shells. In these shells the coloring matter 

 seems to be deposited in two different ways, and at two different 

 periods. In the first process, when the body of the shell is form- 

 ed, the coloring matter is excreted from the glands, in the same 

 way as in other testaceous animals; and it is arranged according 

 to the disposition of the glands on the body of the animals. At 

 this period of the process the shell is only of a moderate thick- 

 ness, and much less than what it afterwards acquires, when com- 

 pletely formed. On the external surface of the shell first formed, 

 another layer is deposited, which is more compact than the first, 

 in some places thicker, and usually variegated with different col- 

 ors. The external surface of the shell being thus completely cov- 

 ered with this second layer, the original colors are concealed; 

 and if the same shell were examined at different periods of its 

 formation, it would appeal- like two distinct species. The organs 

 which are employed by the animal in the production of this second 

 layer of shell, and set of colors, are two soft, membranaceous 

 wings, which being protruded from the opening of the shell, com- 

 pletely cover the whole of its external, convex surface. These 

 two wings, which are quite distinct from the glandular structure 

 about the neck of the animal, which is situated a httle lower, are 

 also provided with glands, which furnish coloring matter, usually 

 different from that which is furnished by the glands of the neck 

 and it is the upper surface of the wings, which is alone provided 



