68 



MOLLUSCA. 



of union being at one side or in the middle. The former 

 are termed close shells, the latter gaping shells. The cha- 

 racter of gaping, so very obvious in the shell, is an index 

 of equally important distinctions which prevail in the ani- 

 mal. In the gapers, the syphons, or the absorbing and 

 ejecting pipes, are two in number, and very long, and fre- 

 quently united. The foot is contained in a sheath, from 

 which it issues at the pleasure of the animal. Besides, the 

 branchiae are always united, and equal in length to the tubes. 

 This character appears, therefore, equally important as the 

 former. It has hitherto been employed in the construction 

 of specific characters merely, rarely of genera. 



When the two valves are of the same size and form, the 

 shell is said to be equivalve ; but when the one valve differs 

 from the other in these particulars, the shell is said to be 

 inequivalve. This character, so obvious and so commodi- 

 ous, is not the index of any peculiar organization of the ani- 

 mal. If employed in the higher divisions, it would separ- 

 ate closely connected genera, and destroy some natural 

 alliances. The inequivalves, however, are for the most 

 part irregular in their growth. The molluscous inhabitants 

 have no lengthened syphon nor foot. 



When we examine the inner surface of bivalves, we ob- 

 serve some spots of a different colour and lustre from the 

 general surface. These are the places to which the muscles 

 adhered, which connected the animal with the shell, and are 

 termed muscular impressions. They are either separate 

 and lateral, subcentral, or simple, or composite. This cha- 

 racter was long employed by conchologists in their specific 

 distinctions, and sometimes in the formation of the genera. 

 It has been more recently employed by Lamark, as a character 

 of the first importance in the division of the bivalves. He 



