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 
branchize 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 eguivalve ; but when the one valve differs 
from the other in these particulars, the shell is said to be 
enequivalve. 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 
