Let none be discouraged by the number of generic distinctions proposed and adopted in modern times; for if well defined, they will be found to facilitate rather than encumber the science. The knowledge of species must be the foundation of every system, and the greater their number, the more necessary it becomes to subdivide them; if, for instance, all the species now known were to have been included in the 50 genera of Linnæus, a single genus would have contained many hundreds of incongruous species, in which case it would be much more difficult to remember them, than if they were to be divided into a far greater number of genera. Every well marked division, however arbitrary its limits, tends to simplify the subject, and to facilitate the researches of the student. NATURE OF THE SCIENCE. Conchology is the study of shells, viewed and described as to what they are either in themselves, or in relation to the soft, inarticulate animals which produce them, and of which they form a part. These animals are called MOLLUSCA, and perhaps the best general description of them will be found in De Blainville's "Manuel de Malacologie et de Conchyliologie." The following is a translation, "Animal in pairs, the body and its appendages soft, inarticulate (not jointed), enveloped in a muscular skin, commonly called the mantle, which is extremely variable in form, and has developed either within or upon it a calcareous portion, consisting of one or several pieces, commonly called a SHELL." The term Mollusca was formerly restricted to those soft animals which were destitute of shells, although possessing in other particulars, the characters described above, and it was used in order to distinguish them from the TESTACEA, which were covered or internally supported by calcareous parts. In the system of Linnæus, the soft portions are first arranged under the general designation of "Vermes Mollusca," and described without regard to the presence, absence, or character of the shells; and then the shells are separately characterized under the appellation of "Vermes Testacea," without any further notice of the animal, than an indication of the genus to which it belongs; thus the animal of Cypræa is said to be a Limax, and that of Tellina a Tethys. The nearest approach to correctness, and the most philosophical method of study will be found in the modern system, adopted by Lamarck and his followers, of observing these animals as a whole, and arranging them according to the assemblage of characters which they present; of course taking into consideration the existence or non-existence, form and structure of the shell, on the same principle, which, in arranging the vertebrated animals would lead us to study the hair, hoof, nails, claws, &c. as well as the other parts.