ON MOLLUSCA. 259 
absolutely in the same vertical plane, there results a discoid 
shell, compressed from right to left, the summit of which can- 
not be visible but in the same direction, and the axis of which 
is altogether equally transverse. These sorts of shells the 
French name enroulées, and we volute (revolute). A rigorous 
example may be taken from the argonaute and neighbouring 
genera, but not in the planorbes, which are really subvolute. 
The principal differences presented by this sort of voluta- 
tion, or rolling, consist in its greater or less perfection; we 
name arched, the shell which only presents an arching more 
or less considerable, as in certain species of belemnites. 
Curved, that in which the body begins to be much more 
curved, as ammonoceros. 
Semi-volute, that in which the turns of the spire, which we 
call whorls, do not touch, as in spirula. 
Volute, when the whorls touch, as in the true ammonacea. 
And finally, very much voluted, the species in which the 
whorls penetrate each other reciprocally, so as that the last 
whorl or circumvolution conceals all the others, and the 
aperture is modified by it, as in some nautili. 
If, on the contrary, the rolling of the spiral cone is made 
transversely, or from left to right, this constitutes what are 
called énvolute shells. | 
In these species the base of the shell is almost as long as 
the latter, as well as its aperture, and the axis of volution is 
longitudinal. There are in reality scarcely ever any shells 
completely involuted; those which approach the most to 
being so are the cypreade and the ovule. Sometimes the 
shell does not make a complete whorl, as in the bullez, and 
then the aperture is as broad and as long as the shell itself. 
Finally, the great majority of univalve shells are interme- 
diate to both these arrangements; that is to say, that the body 
of the shell is the result of an oblique volutation from right to 
left, and from bottom to top, if we consider it from the base 
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