CORNUA AMMONIS, in natural history, are fossil
shells, of which a pretty full account is given in the
Encyclopædia. See CORNU Ammonis and SNAKE-
STONES. It was observed in the last of these articles,
that few, if any, of these shells are known in their re-
cent state, or as occupied by the living animal; but
some authors have asserted, on the authority of Lin-
næus, that ammonites, with shells similar to all the va-
rieties of the fossil ones, are yet found alive in the depth
of the sea. We are much inclined to embrace this opi-
nion; but it has been controverted by M. de Lamanon,
who accompanied La Perouse on his voyage of dis-
covery, by such arguments as we know not how to an-
swer. This unfortunate naturalist (see LAMANON in
this Supplement) allows that there are still in the sea
living cornua ammonis; but he thinks that they are in
very small numbers, and materially different from the
greater
greater part of the fossil ones. According to him, these last ought to be considered as a race, formerly the most numerous of all, of which, either there are no descendants, or those descendants are reduced to a few degenerate individuals. That there are no living animals with shells of the very same kind with some of the fossil cornua ammonis, the following observations he considers as a sufficient proof.
"The fossil shells are very light and thin, whereas the shells of those animals that live in very deep water are always thick and ponderous; besides, the form of the fossil cornua ammonis points out to us, in some measure, the organization of the animal which inhabited it. The celebrated Jussieu proved, in 1721, that there existed a very close analogy between the ammonite and nautilus (A). It is well known that the nautilus, by filling or emptying a part of its shell, has the power of remaining stationary in any depth it pleases: the same was doubtless the case with the ammonite; and if this species still abounds in the sea, it would surely be occasionally discovered by sailors.
"The waves also would throw fragments of it on the shore; fishermen might sometimes entangle it in their nets; or, at least, there would be fragments sticking to the lead of the sounding line when ascertaining great depths. It may also be added, that if the ammonites never quitted the abyss of the sea, those which are found petrified would not be constantly met with on the same level, and in the same bed, as those shell fish that only inhabit the shallows. There are, however, found in Normandy, Provence, Touraine, and a multitude of other places, ammonites mixed with turbines, buccina (whelks), and other littoral shells. They are found, besides, at every degree of elevation from below the level of the sea to the summits of the highest mountains. Analogy also leads us to suppose, that Nature, who has given eyes to the nautilus, has not refused them to the ammonite; now what use could these be of if they remained confined to those depths which the light is unable to penetrate?
"The extinction of the ancient race of ammonites is therefore an established fact, which no rational supposition can destroy; and this fact is undoubtedly the most surprising of any that is presented to us in the history of aquatic animals. The discovery of a few living species of cornua ammonis does not destroy the truth of this, for these ammonites are very different from those which are found petrified. They are extremely rare, and cannot be looked up to as the representatives of the old ammonites, so varied in their species, and the number of which in the ancient ocean was probably far more considerable than that of all the other shells besides."
"To every univolve shell, rolled in a spiral, so as that a horizontal plane will divide it into two equal parts, formed of united spirals, and bearing a certain proportion to each other, our author gives the name of an ammonite. "I thought it absolutely necessary (says he) to ascertain the precise meaning of the term ammonite, pre-