170 Conchology. 



nally colored, the fluids secreted from the body of the animal are 

 of the same color, and they take the place of those which are 

 usually white, or of a pearly nature, as is observed in many oth- 

 ers. The nature of these internal layers is always obvious; for 

 if they are not white, they exhibit everywhere an uniform color, 

 and never variegated, like what appears externally. By removing 

 with a file any part of the external surface of the shell, the layers 

 which appear immediately under the surface, as those which have 

 been furnished by the body of the animal, while those on the sur- 

 face itself, usually more variegated than the rest, owe their forma- 

 tion to the vessels about the neck, and have been formed in the 

 way already described. 



The growth of shells, being proportioned to that of the inhabi- 

 tant, proceeds almost imperceptibly. In most shells, however, 

 it is easy to distinguish the different additions which they have 

 received; for they are marked on their convex surface with dif- 

 ferent eminences which are parallel to each other, similar to lines 

 of different degrees of depth, which give the shells a fibrous struct 

 ure. These elevations called strioi^ may be traced through 

 the whole of the shell in bivalves, and in the longitudinal direction 

 of those which have a spiral form. From the slightest observa- 

 tion of the manner in which shells are formed, it is easy "to see 

 that they can receive no addition, without leaving, in a greater or 

 less degree, some trace of these irregularities; for every small ad- 

 dition of testaceous matter which is made, must be attached to 

 the old part of the shell, which consequently must be more elevat^ 

 ed than the former, whatever be its thickness, when the enlarge- 

 ment of the animal requires the formation of the latter. Thus the 

 shell will be marked with a great numb'^r of these striag, parallel 

 to each other, which may be distinctly seen on many different 

 species. 



Every shell has usually some of these eminences at greater dis- 

 tances, and more elevated than the others. By these the differ- 

 ent periods when the shell ceased to increase, or rather those 

 when its growth was interrupted, are marked; and they have some 

 degree of analogy with the different shoots from the branch of a 

 tree. The heat of summer or the cold of winter interrupting the 

 growth of the animal, at least among such as are testaceous, which 

 hve on the land, or inhabit rivers in temperate regions, the shell 

 is not enlarged in extent during these seasons. It is otherwise, 

 however with regard to its thickness, for there is continually exud- 

 ed from the body of the animal, small quantities of fluid, which 

 increase its thickness. Hence it is when the shell begins to in- 

 crease in extent, the edge to which the new portion is cemented, 



