From a process exceedingly similar, we may suppose the formation of the warted protuberances and tubercles to have pro ceeded; correspdnding fleshy projec tions, existing on that part of the surface of the animal to which this part of the shell has been applied, having been the moulds, as it were, on which these have been formed. Whilst these fleshy protu berances have continued, these processes have remained hollow ; but where the fleshy protuberances have been diminish ed, these processes have become partly filled up ; and where they have been re mm ed, these have become entirely solid, from the deposition of additional shelly matter from the surface of the animal, the existence of the cavities being necessary no longer than the continuance of the fleshy processes themselves.
The production of the spinous pro cesses, so frequent on many shells, ap proaches still nearer to that of the ribbed protuberances, since they have been form ed by little, long, tongue-like processes, with which the reek of the animal is be set, and which have served in the same manner to produce these spines, as the neck of the animal has served to form the bordered edge of the shell, by an addi tion of the testaceous matter at each time the neck of the animal, at certain periods, passed out or returned into the shell. The confirmation of this being the mode of their formation is yielded, by the cir cumstance of these spines being ranged in lines, at equal distances, on the ribbed protuberances, or varices of Lintizus, the formation of which has not only been si milar, but simultaneous.
The production of the channel, or gutter, (the cauda of Litinzus,) depends on circumstances extremely similar to those on which has depended the forma tion of the various parts already noticed. All those animals whose shells have this termination, are endued with an organ of a cylindrical form, capable of being con tracted and extended, so as to allow the animal to explore its path, or to attach itself to neighbouring bodies. By the frequent employment of this organ, and by its necessarily accompanying frequent alternate contractions and extensions, its surface, which possesses the property of exudine a testacemis substance. must at
each passage and return contribute to the formation of this canal, which would serve the purpose of a sheath to it.
By an ingenious application of these principles of Reaumur, Brugulere and Brogniart (" Bulletin de la Societe Philo mathique," No. 25.) have very satisfacto rily explained the production of the seve ral fissures, striz, grooves, and other mo difications of the forms and surfaces of dif ferent shells. The author last mentioned has shown, that, in some instances, the or gan by which some changes have been produced in the later formed part of the shell was not acquired by the animal until it had obtained its full size. By an appli cation of these principles to bivalves, and even to multivalves, their peculiarities of form have also been very ingeniously Sc. counted for. It has been found, that, in bivalves, the part of the animal which is termed the mantle is the organ on which the peculiarity of the form of each chiefly depends ; it answering in this respect to the collar, or neck of the animal, belong ing to univalve shells.
It does not seem necessary here to dwell on the labours of those, who, previous to the time of Linnzus. had endeavoured to dispose of shells under the most compre hensive and distinctive arrangement. Whatever had been done before with this view gave way to the more lucid and cor rect disposition which Linnzus adopted; but this is certainly by no means the ne plus ultra : other divisions appear in some instances deserving of adoption, and new genera require to be admitted. It is, therefore, intended to give here Lin. ideus's arrangement of the genera, omit. tins the species; afterwards to give that which has been adopted by Du Bosc, agreeable to the discoveries and obser vations of Lamarck, Brugulere, and others ; and to offer a few explanatory remarks, for the purpose of showing the propriety or impropriety of the de viations from the arrangement of Lin nzus.