246 SUPPLEMENT 
Lastly, the consideration of the shell alone ought not to be 
regarded as altogether of no value, or useless, especially when 
we review in succession the differences according to their 
degree of importance: 1. the number of pieces which enter 
into its univalve, sub-bivalve, or operculate, bivalve, tubi- 
valve, and multivalve composition; 2. the position on the 
body of the animal, dorsal, as in all the cephala, dorsal and 
ventral, as in a small number of cephala and acephala, or 
finally bilateral, as in all the lamellibranches ; 3. the indices 
of its relations with the respiratory apparatus, that is, the ex- 
istence of an emargination or a tube at the anterior extremity 
of the aperture in the univalves, or of an hiatus more or less 
considerable of the posterior extremity in the bivalves; 4. the 
indices of its relation with the muscular system of the animal, 
simple in a great portion of the univalves, but merely visible 
in the patelloides and otides, more or less complex in the 
bivalves, and formed, as we have already seen, by one, two, 
or even many impressions of the adductor muscles, one, two, 
or a greater number of impressions of the retractor muscles of 
the foot, by the abdominal ligula, an index of the attachment 
of the edges of the mantle, and finally, behind, by that of the 
tubes of respiration; 5. the symmetrical or non-symmetrical 
form which involves the similitude or dissimilitude of the 
pieces in the bivalves; 6. the form of the aperture in the 
univalves, the manner in which each edge and the columella, 
or its vitreous depot, contribute to form or modify it; 7. the 
ligamentous system, or the dovetailing, so to call it, of the 
two pieces of a bivalve shell, that is to say, those of the hinge 
and of the teeth which compose it, observing that each 
genuine species has a peculiar system of indentation on the 
edges of the shell; 8. the consideration of the existence or of 
the absence of an operculum in the univalves, of its structure, 
form, &c.; 9. the total form of the shell, the proportion of 
the spire, and of the aperture in the univalves, the direction 
