Conchology. 173 



with this glandular structure. This surface, when this part of the 

 animal is protruded from the shell and extended over it, comes 

 in contact with the external surface of the latter. Hence it is, 

 that these membranaceous organs deposit on the first formed and 

 colored layers of the shell, new layers of testaceous matter, which 

 is differently colored, and diversified with entire spots, either cir- 

 cular or in a waved direction, which are sometimes of a more vi- 

 vid tint than that of the ground, or white upon a dark ground, or 

 brown upon a yellow ground; or are composed of straight fines, 

 or curved or interlaced with each other, reddish brown, yellow 

 or white; on different colored grounds, or in dots or points, whose 

 shades and arrangement are not less diversified. 



This mode of the formation of the external layer of porcelain 

 shells, has been proved by the actual observations of some natu- 

 ralists. In some species, a longitudinal line of a paler color is 

 ob=erved on the convex surface of the shell. This is ascribed to 

 the junction of the two wings of the animal, where a smaller quan- 

 tity of coloring matter has been deposited, or where the shell has 

 been less completely covered with the protruded part of the ani- 

 mal. But the existence of this second layer is still more distinct- 

 ly proved by mechanical means. 



The external layer may be removed by means of a file, and the 

 shell restored to its original state; and then the colors which it 

 first received are brought into view. This circumstance is still 

 farther demonstrated by an attentive examination of different spe- 

 cies of shells, and particularly the Cyprcea argus. In examining 

 this shell, there are observed under the external layer, which is 

 of a yellow color, some slight traces of four transverse bands of a 

 brown color, which surroimd the shell, and which must have been 

 formed previous to the more superficial yellow layer. By a more 

 minute examination, it will appear that the circular spots, with 

 which the external yellow layer is marked, have been posteriorly 

 formed to this layer; and finally, on the four turns of the spire 

 forming a slight projection at the base of the shell, there are some 

 brown, circular spots, which are quite superficial, and which 

 sometimes include two turns of the spire, which could not hap- 

 pen if the yellow color had not been prior in its formation to these 

 circular spots. If the coloring matter of which these spots are 

 composed had been deposited at the time that the different parts 

 of the spire were formed, one spot could not have included two 

 turns of the spire at the same time. 



This effect of communicating a new set of colors to the exter- 

 nal surface of the shell, is not the only one which is produced by 

 the membranaceous structure of the animal which inhabits the 



