A peculiar appearance is observable on the surface of many of these fossils, which depends on the peculiar form of the septa which separate the chambers of the shell. These septa in the nautili are smooth, and terminate at the surface of the shell in a straight line ; but in the Cornua Ammonis they become undulated as they extend outwardly ; and in some so much so as to form, on the outer surface, deeply crenu lated lines, giving the appearance of folia ceous sutures. When the cavities of the shell have become filled with stone, and the septa just mentioned have been re moved, as is frequently the case, by some chemical agent, the casts formed in the chambers separate, each forming a curi ously figured stone ; these separate casts have been termed spondilolites. (Plate H. fig. 4.) By the junction of these are formed the foliaceous sutures above-men tioned. The Cornua Ammonis were for merly called serpent-stones ; the appear ance which they yield of a serpent coiled having led the vulgar to consider them as petrified serpents.
The fossil Cones are very few when compared with the numerous species known in a recent state ; the same may be also said of the Cyprem. In both these genera the species are mostly made out more from the colour and the markings of the shells, than from the peculiarities of their form ; but in the fossil shells the co lours no longer exist, and of course the species in these can very seldom be pre sumed. The fossil Volutes, as far as can be judged from their form alone, differ generally from the recent species. With respect to the genus Buccinum, Strombus, and Murex, the number of species of the fossil shells do not appear to equal those which are known in a recent state. This is the case also, in a still greater degree, with the genus Trochus. The fossil shells of the genus Turbo are pretty numerous, and some of them very closely resemble those of known recent species. One fos sil shell of this genus is very remarkable for its vast size, being upwards of a foot in length. The cast of another species is so large as to weigh four or five pounds. Nothing like this occurs with respect to the species of the genus Helix : the fossil shells of this genus very much resemble those which are recent, and are not found of any considerable magnitude. The fos sil shells of the genus Nerita by no means display so many species as the recent ; but some of the fossil species far exceed the recent in size, and one in particular is twelve times the size of any known recent species. Of the genus Haliotis, it is not positively determined that a single shell has been seen, which could be considered as fossil. Fossil shells of the genus Pa telta are by no means common. Several species have, however, been found in France, in a state of excellent preserva tion. Some few also have been found in the cliffs at Harwich, and others, of a dif ferent species, imbedded in the lime-stone of Gloucestershire. Dentalia, apparently similar to existing species, have been found in Hampshire, and in some parts of France and Italy, exceedingly well pre served. In Italy, also, have been found
specimens of Serpulx, very similar to those which are known recent ; but others have been found in France exceedingly different from any known recent species.
The Orthoceratites, a lapidified conical or cylindrical chambered shell, the septa dividing the chambers of which are perfo rated like those of the Nautilus, is a ge nus of which not a species is known in a recent state, excepting the microscopic specimens found by Plaucus in the sand of the Riminian shore. Much is wanting to complete the history of this fossil, since from the state in which the specimens have in general been found, very few, or perhaps none, have been obtained perfect. Authors have divided them into those which are straight (Plate II. fig. 8.), and those which have a spiral termination, the latter of which are considered as fossil shells of the Nautilus litmus ; but the ex traordinary disparity of size is sufficient to shew that they can hardly be considered of the same species, the recent shell being seldom more than an inch in length, whilst the fossil is described as being sometimes the size of a man's arm.
The Belemnite (Plate II. fig. 7.) is a spa. those radiated stone, generally conical, but sometimes possessing a fusiform fi gure, and contains, in an appropriate cavi ty at its larger end, a smaller calcareous body (alveolus) which has evidently been a concamerated shell, the septa of which are pierced like those of the preceding fossil. These fossils are from an eighth of an inch to two inches in thickness, and from an inch to a foot and a half in length.
They are sometimes found imbedded in chalk or limestone, and sometimes in pieces of flint ; but they are most fre quently detached fforn their matrix. Va rious have been the opinions respecting this fossil; some have considered it as the horn of a narwhal, and others as a concre tion formed in the pennicilla marina, or in some shell of the dentalium kind. Some have even supposed it to be of vegetable origin, whilst others have considered it as entirely belonging to the mineral king dom. But that the Belemnite originally existed in the sea, is evident from its be ing commonly found with the remains of the undoubted inhabitants of the ocean, and that it is of an animal nature, is ren dered evident by its structure. Among the concameratecl fossil shells may be placed the Helicites, or num mular, or len ticular stones. These are round flattish bodies : but in general of a lenticular form, both sides possessing a slight de gree of convexity. On each side are some times seen traces of its internal structure and of its spiral formation ; whilst some times these appearances appear to be con cealed by a thicker covering. Various opi nions have been entertained respecting their origin, but no doubt can exist of their having existed in the ancient ocean as a spiral chambered shell, and of their being one of those species of animals which are now lost.