With respect to the state in which fos sil shells are found, it is necessary to re mark, that, in some situations, shells which have been buried for ages, by the natural changes which the surface of the earth has undergone, are found very little changed, except from the loss of colour, and having been rendered extremely fra gile; that in othersituations the substance of the shell has been so injured, as to be reduced to very small fragments, and even to a very fine powder, leaving in some instances a stony, correctly mould ed, cast of the cavity of the shell ; that very frequently the substance of the shell is entirely altered, having become a cal careous stone, or a silicious or pyritous mass, and that the shells of a fbrmer world are frequently found in masses of marble, which is called lumachelli, or shelly marble.
Of the Multivalvcs, the chiton does not appear to have been found in a mineral ized state ; and although several species of Lepas have been found in a mineral state, they are by no means frequent fia ,sils. Lepas anserifera is said to have been found fossil, as well as Lepas diade ms ; these must, however, be exceeding ly rare fossils.
Fossil shells of the Pholas are by no means common ; the Pholas crispata has been, however, found among the Harwich fossils.
Fossil bivalves are very common fossils; they are, as might be expected, very sel dom found in pairs, except when united by a lapideous mass, which prevents the examination of their hinge, or their inter nal structure, which in many fossil shells are objects highly worthy of examina tion.
The Mya pictorum is described by So lander as existing among our Hampshire fossils : a fossil mya of three or four inches in length, is found also in the rocks near Bognor. Remains of the solen and of the solen ensis, have been found at Harwich, and a small fossil shell, nam ed by Solander solen ficus, has been found between Lymington and Christ church.
Fossil shells of the genus Tellina, as well as of cardium, mactra, donax, Venus, spondylus, chains, arca, and particularly ostrea, have been found of many spe cies. But no bivalve exists as a fossil in such prodigious numbers, and in such various species, as those of the genus Anomia. These shells are characterized by the beak of the largest or under valve, which is perforated, being greatly pro duced, rising or curving over the beak of the smaller or upper valve. Anomia lacunosa (Plate II. fig. 1.) is one of the most abundant of these species. They are found in considerable quantities in different parts of England, particularly in Lincolnshire, Warwickshire, and Glou cestershire. Anomia terebratula, (Plate H. fig. 2), is another fossil of this genus, which exists in different counties in this island, in great abundance.
Of the genus Mytilus several species are known as fossils, some: of which ap proach very near to those which are known recent : one in pariicalar appears to differ very little indeed from Mytilus modiolus. Fossil shells of the genus Finns, in any tolerable preservation, are not frequently found : the shells are in general so fragile as to render it very difficult to obtain them tolerably perfect ; or so that but little information can be yielded respecting the species to which they belong.
No fossil shell appears yet to have been found which can with certainty be placed under the genus Argonauts. But of the genus Nautilus, specimens are very frequent. These have been found in several parts of this island : some very fine specimens have been found at Lime in Dorsetshire, in different parts of Wilt shire, and at Whitby in Yorksbire. The finest specimens are perhaps found in the neighbourhood of Rath, and in the Isle of Sheppey in Kent, at which latter place they are found exceedingly large, and still retaining a resplendent pearly shell. (Plate H. fig. 3.) The Corms Ammonis, which, if we ex• cept the extremely minute shells of this kind which have been seen by Planck's, and others, in the sea sand on the 'Vene tian shores, may be said to be only known to its in a fossil state Like the Nautilus, the Corm, Ammonis is divided into compartments, by regular ly disposed partitions, and these parti tions are perforated, as are those of the Nautilus, although it is by no means easy to point this out, except in very few spe cimens.
There are none of the fossil shells, ex cept perhaps the Anomix, which can vie in the variety of their species with the Corns Ammonis. The shell of some is perfectly smooth over its whole surface ; in others smooth at the sides, but ridged or beset with spines at the back ; and others, though smooth at the side, are crc nulated at the back. The species most commonly met with have the shell vari ously ridged ; some with small close strife, and others with large and round ridges. In some the ridges are single, in others bifurcated, and in others trilii•cated. In some, and these are least common, the shell is tuberculated : these tuberculm differing considerable in different species, in their size, form, and disposition. The different species proceeding from the in. tcrmixture of all these varieties, it must be obvious, must be exceedingly numer ous : Scheuchzer was able to determine the existence of one hundred and forty. nine species. The difference of size ob servable in these fossils is not less re markable than the variety of their forms, some being found not much larger than the head ola pin, whilst others have been found as large as the top of a small table.