428 



THE THORNY CLAM. 



by the presence of the foreign body forces the/mollusc to deposit the nacreous secre- 

 tion upon the intruding substances, and after a while the shot are covered with layer 

 upon layer of pearly substance, the thickness of the coating depending upon the 

 length of time occupied in the construction. 



The same ingenious people are also accustomed to make curious little pearl-covered 

 josses, by stamping them out in thin bell-metal, slipping them into the shell, and leav^ 

 ing them between the valves until they are sufficiently coated with pearl. The ex- 

 ample from which the illustration was taken is in Mr. Cumming's collection together 

 with several others. 



IN the THORNY CLAM, a curious member of another family, the shell is covered with 

 long and branching projections, something like the horns of a young roebuck. All the 

 Clams are natives of the warmer and tropical seas, especially among coral reefs, and 



BRITISH PEARL-MUSSEL. 't/n/o margarltiferus. 



CHINESE PEARL-MUSSEL. D/psas pHcatus. 



their color and shape are extremely variable. Mr. Broderip writes of them as follows : 

 " The shells are attached by their external surface to submarine bodies, such as coral 

 rocks, and shells have been observed at depths varying from points near the surface 

 to seventeen fathoms. These shells appear to be subject to every change of shape, and 

 often of color, that the accidents of their position may bring upon them. Their shape 

 is usually determined by the body to which they are fixed ; and the development of 

 the foliated laminae which form their general characteristic is effected by their situa- 

 tion ; and their color most probably by their food, and their greater or less exposure 

 to light. The Chama that has lived in deep and placid waters will generally be found 

 with its foliations in the highest state of luxuriancy, while those of an individual that has 



