ix or rather according to the necessities of the animal. Among the spiral shells these varying forms are the result of an obliquity in the mode of growth, caused by a greater development or increase of one side of the opening, whereby an heliciform direction is given to the shell in the process of enlargement.It is, perhaps, needless to say, that the mode of increase or growth of the shell is by the successive deposition of calcareous matter secreted by the mantle, and deposited on the margin of the aperture, and moulded as it were upon the soft body of the animal, by which means the impress of every peculiarity is formed, and permanently fixed in the calcareous covering. It may, however, be observed, that some of the exterior ornaments of these shells are formed by an extension or protrusion of the mantle, whereby a fimbriated, and sometimes a spinose, varix is formed upon the margin of the outer lip, producing many varied and beautiful ornaments upon the spiral sheil, which, however, have no corresponding characters upon the soft body of the animal. It is from these appearances alone, that presumed specific distinctions can be determined ; and the characters drawn from them, although they may be considered as somewhat extrinsic to the animal, are the only means afforded to the palaeontologist for the determination of species.From the recent researches of Mr. Bowerbank and Dr. Carpenter, in their Microscopic Examinations into the Structure of Shells, it would appear that there is a more intimate connexion between the animal and its envelope during its lifetime than could be imagined by the supposition of a simple addition of earthy matter deposited upon its cuticle ; and that this strong covering is an organized body, and part and parcel of the animal itself. Important as are those observations and suggestions, which bear considerably on generic groups, and by which, perhaps, even minor distinctions may hereafter be pointed out, the specific determinations referred to in the present monograph have not been submitted to such tests, but are merely drawn from the various external differences existing in the shells, and which variations in structure are generally considered as resulting from some internal economy of the animal.This mode of determination may, perhaps, be correctly applicable only to shells in perfect condition ; whereas, some of those figured in this work have undergone considerable alteration, either by the loss of colour and animal matter, or by decomposition of the outer coating, while many of them have sustained mutilation by the mechanical action of the waves. These accumulated difficulties are serious impediments to accurate comparisons ; and the identifications in many cases must be looked upon only as approximations : a correct Marine Fauna of this period will, it is to be feared, be for many years to come a desideratum. CoNOVULUS,* Lam. 1812. Melampus. Montf. 1810. CoNOVULA. Fervs. 1819. Makinula. King. 1831. CoNOVULUM. G. B. Sow. jun. 1841. AuEICCLA. Lam. VoLUTA v&pec.) Linn. Gen. Char. She...
