TESTACEA. 57 



CHAPTER III. 

 BIVALVE SHELLS. 



THE preceding chapter being confined to miscellaneous general illus- 

 trations of the nature of a few inhabitants of univalve shell*, the present 

 one shall be devoted with corresponding brevity to some observations on 

 a few of the bivalves, selecting for the most part those which are not 

 difficult to be obtained. 



But llu'ir features are not equally prominent. The former are con- 

 stantly exposed. Their feeding and their breeding is generally patent to 

 view. Both survive in confinement. The naturalist has to study lit tit- 

 more than the season of the year to satisfy himself. It is different with 

 the others. The food of scarcely any of them is known. Even under 

 the most favourable circumstances, the spectator is denied a complete 

 view of the inhabitant of the bivalve shell. None of its faculties are 

 sensibly exercised, with rare exceptions, and if a scanty margin or the 

 instrument of progression chance to have a temporary and partial expo- 

 sure, the whole is suddenly withdrawn on the slightest alarm, while the 

 opaque testaceous covering closes permanently over them. 



Thus, the form, the habits, and the nature of the tenant of the 

 bivalves are ascertained with much less facility than those of the others. 



Nevertheless, conchologists have reared a system on no other foun- 

 dation than the external covering, much easier, indeed, than if founded 

 on the edifice and its inhabitant combined. What should we say of a 

 discussion on the figure, proportions, and properties of a savage tribe. 



H 



