By a law tacitly admitted, but universally recognized, for the sake of securing to a name its intangibility, no two genera in the same kingdom of nature may be named alike. There is, therefore, if this rule be observed, no fear of similar names coming into collision in the same province, and thus producing confusion. A ready means to prevent the possibility of such mischance is the admirable work which has been published by Agassiz, with the assistance of very able coadjutors, in the ‘Nomenclator Zoologicus,’ which is a list of all the generic names extant in zoology, exhibiting what names are already in use either appropriately or synonymously in this great branch of the natural world, and if this work receive periodically its necessary supplements and additions, no excuse will remain for the repetition of a name already applied. The most defective character in this laborious work, is the frequent incorrectness of its etymology of the names of genera. It would be, perhaps, without such aid, too great a labour to require of the describing naturalist, or it might not be otherwise even practicable for him, to ascertain whether the generic name he purposes to impose be, or not, anticipated. The penalty of its being superseded is understood to attach to the imposition of such a name, for the alteration may be made with impunity, and thereby it becomes degraded to the rank of a mere synonym. Nomenclature has thus, by the happy invention of Linnæus, been made a matter of the greatest simplicity, conciseness, and lucidity, and to him, therefore, our gratitude is due. An indispensable branch of nomenclature is _Synonymy_, which, briefly, is the chronological list of the several names under which species or genera may have been known. This diversity of names has originated in several ways,—from indolence, or ignorance, or excessive refinement. The views of systematists will differ in the collocation of creatures; hence, sometimes what had been previously divided will be recombined, or divisions into further groups be made of what had been before united. Both processes will necessarily produce synonyms; the recombination of what had been separated reduces the names of such groups to the rank of synonyms of the old one from which they have been disjoined. In the latter case the old name will be retained to the typical species merely, and be also made a partial synonym of the names of the new generic groups: or, indeed, it may happen that the same creature has been described generically, unknowingly, by two different persons, about the same time. By another recognized rule in nomenclature, the ‘law of priority,’ the name given by the first describer is accepted, and the other consequently falls to the condition of a synonym.