ON MOLLUSCA. 195 
veil of greater or less extent; at other times we find that the 
lips are prolonged into a sort of cupping-glass, inthe bottom 
of which is the proboscis, as in the cones; finally, we often 
remark that they are prolonged in the same way, but acquir- 
ing a tolerable degree of thickness, and there results a probos- 
cidiform muzzle, as in a great number of species of the family 
of cyclostomata, an organ susceptible of contraction or elonga- 
tion, but without ever being able to re-enter into the buccal 
cavity, which distinguishes it from the true proboscis, of which 
we shall presently speak. . 
Within these contractile lips, and sometimes provided with 
some small specific muscles, are often found corneous or cal- 
careo-corneous organs, to which the name of jaws has been 
erroneously applied; these in fact are genuine teeth produced 
from the skin, which they cover, and the structure and mode 
of formation of which are altogether analcgous. 
We rarely find two of these teeth acting vertically one upon 
another, as in sepia, or horizontally, as in tritonia. In such 
cases they are surrounded at their bases by a thick circular 
muscle, which presses them strongly one against the other, 
after they have been separated by the action of the levator 
muscles of the upper, and the depressor of the lower. 
In a much greater number of cases there is but one upper 
tooth in the form of a comb, curved and denticulated on the 
edge. It is then pretty nearly immoveable, and the tongue, 
of which we shall presently speak, acts upon it. This is what 
may be seen in all the animals of the family limacines, in that 
of lymnza, of auricula,.and even of patella. 
In a much greater number, again, there is no trace of true 
marginal teeth, as in all the cephalous mollusca, provided 
with a proboscis, and in the entire class of the acephala. 
At the lower face of the buccal cavity there often exists a 
swelling more or less considerable, which has been compared 
with some justice to that which forms the tongue in the boned 
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