404 SUPPLEMENT - 
there are never lined with a calcareous deposit, as in certain 
neighbouring genera, the reason of which may be very well 
conceived, as the mantle is entirely covered by the shell. The 
movements of the solens are usually confined to an ascent or 
descent in their hole, which is sometimes two feet in depth. 
This movement is doubtless produced by the action of the 
foot, which becomes attenuated at its extremity for the pur- 
pose of descending, or being widened, assumes a resting point 
from which to ascend, and cause the tube, and even a part of 
the shell, to pass the orifice of the hole, at the surface of the 
sand, and to rise more or less in the water which covers it. 
It does not seem probable that the animal ever comes forth 
altogether from this retreat of its own accord, though we can 
very well conceive it possible that it should do so. But it is 
certain, according to the observations of Reaumur and Adan- 
son, that if by any cause it has been drawn out of it, it can 
enter there again. The first of these observers has described 
the mode in which the animal proceeds for this purpose. 
Curving and sinking the extremity of its foot, it commences 
by raising its shell more or less obliquely to the horizon; 
then a new impulse, by straightening the foot, begins the 
sinking of the shell, at the same time that it makes a still 
shorter angle with the horizon. The same action renders it 
vertical and alittle sunk. Then it extends its foot as directly 
as possible, and giving it. the form of an angle, by then draw- 
ing it back, the shell to which it is attached descends. By 
repeating these motions, it sinks very quickly. The ascent, 
on the contrary, is made by drawing back the foot strongly, 
and widening it much; the resisting point is on the swell of 
the foot, and the motion takes place in the shell upwards. 
We know little more concerning the natural history of the 
solens. Aristotle, however, tells us, that these animals seem 
to hear, when a noise is made near them. The only ground 
for this opinlon is, that if a sudden and somewhat loud noise 
