84 THE SEA 8HOEE 



pack the dry shells with cotton wool, and then fasten the opercula 

 to the wool by means of a little gum tragacanth or acetic glue. 



Bivalve shells should, as a rule, be closed while the ligament is 

 still supple, and kept closed until it is quite dry, when the valves 

 will remain together just in the position they assume when pulled 

 together by the living animal. The shells of the larger species 

 may be conveniently kept closed during the drying of the ligament 

 by means of thread tied round them, but the very small ones are 

 best held together by means of a delicate spring made by bending 

 fine brass wire into the form shown in fig. 44. 



There are many features connected with the internal structure 

 and surface of the shells of molluscs that are quite as interesting 

 and instructive as those exhibited externally ; hence a collection of 

 the shells intended for future study should display internal as well 

 as external characteristics. Thus, some of the spiral univalve 



' shells may be ground down on 

 an ordinary grindstone in order 

 to display the central pillar (the 

 columella) and the winding 



FIG. 44. SPBING FOB HOLDING cavity that surrounds it, while 

 TOGETHER SMALL BIVALVE SHELLS others, such as the cowries, may 



be ground transversely to show 



the widely different character of the interior. Bivalve shells, too, 

 may be arranged with the valves wide open for the study of the 

 pearly layer, the lines of growth, the scars which mark the posi- 

 tions of the muscles that were attached to the shell, and the teeth 

 which are so wonderfully formed in some species. 



Some collectors make it a rule to thoroughly clean all the shells 

 in their collection, but this, we think, is a great mistake ; for when 

 this is done many of the specimens display an aspect that is but 

 seldom observed in nature. Many shells, and especially those 

 usually obtained in deep water, are almost always covered with 

 various forms of both animal and vegetable growth, and it is 

 advisable to display these in a collection, not only because they 

 determine the general natural appearance, but also because these 

 growths are in themselves very interesting objects. Further, it is 

 a most interesting study to inquire into the possible advantages of 

 these external growths to the inhabitants of the shells, and vice 

 versa a study to which we shall refer again in certain chapters 

 devoted to the description of the animals concerned. 



But there is no reason whatever why some of the duplicate 



