In _Conchology_ many of the genera long established on the Continent, but new to our own collectors, have been characterized and illustrated, as well as specific distinctions defined between shells hitherto considered as varieties; and here it must be observed that so much latitude has been given to the meaning of the term _variety_, that in its general acceptation its definition becomes impossible: our own idea of its true meaning is, a shell possessing one or more characters which are changeable and uncertain, and which consequently will not serve as indications by which it may infallibly be distinguished from all others; _variety_ depends on local circumstances, and affects the size, colour, and greater or less development of the same modification of structure; a _species_ is permanent, its structure always the same though more or less developed, producing and perpetuating its kind, and depending on formation, discernible in youth, and matured in age: we cannot therefore comprehend the contradictory term of _permanent varieties_ in a state of nature (though such occur in domesticated animals), which some authors have used, and which has led to, in many instances, the most erroneous conclusions. It is lamentable to see the opposition which is still made by our own writers against all the modern improvements; yet although Linnaean Introductions to Conchology are constantly issuing from the press, the desire of being acquainted with a more natural and intelligible classification has already appeared; and as we are frequently questioned on the subject, we cannot in this place do better than refer the young student to the valuable article on Conchology contained in the late supplement to the _Encyclopedia Britannica_, the perusal of which will convey more solid information, and less perplexity, than all the Introductions our booksellers can boast of. With respect to the quotations or synonyms, it should be observed that we have in most instances limited them only to original works, all doubtful ones have been rejected, and such only given as have been actually consulted; indeed to this latter cause must be attributed the occasional omission of some, existing in books we had not the immediate power of consulting; our own library is not small; but the difficulty and expense of procuring all the new continental publications, and the impossibility of meeting with them at our public libraries[2], may have sometimes led us into error, and unintentionally to have passed over the discoveries of others.