Page:Land Mollusca of North America (north of Mexico) Vol. I Part 1 i-276.pdf/16

XVI throughout the work to give the views of other authors as well as my own, usually by liberal quotations. It is to be expected that my decisions as to the status of forms may sometimes require revision. Many of the published names belong to the terminology of variation rather than to taxonomic nomenclature, and they cannot properly be used at trinomials.

From the time of Férussac to about 1870, and in some countries later, authors frequently followed their descriptions of shells by terms denoting variations of size, form or color, such as minor, major, maxima, depressa, dentifera, unicolor, alba, fasciata and the like, sometimes prefixed by "var." (hence the feminine form), or more often by a Greek or Roman letter, α, β, γ and so on. They were not intended to denote geographic races or subspecies in the sense these terms are used now, but rather as variations within the species. Often there was no definition other than that implied by the name. J. W. Taylor in his Monograph of the Land and Freshwater Mollusca of the British Isles used the terms variety and subvariety for variations of this character; and examples are given in this volume, pages 7, 8, 14 and 30. Identical terms were frequently used for the variations of successive species of one genus. It would obviously be improper to apply the rules governing binomial and trinomial names today to this variation terminology of the Pfeifferian era. The attempt to incorporate it into our nomenclature would raise many perplexing questions of priority and of adequacy of definition. Moreover their introduction would prejudice a number of well-established specific names.

Genera.—Although the classification of land shells has occupied much of my time for many years, the systematic position and significance of the Ammonitellinae, and Oreohelix, and much of the arrangement of the Polygyridae, have been worked out for the first time in this volume. Except in the subfamily Polygyrinae, where genera and subgenera rest mainly upon characters of the shell alone, the genera of our helices are largely based upon internal structure, which has proved to be remarkably constant in long series of species dissected. In almost all cases there are correlated shell characters by which the genera are readily recognized.

Key to new names in Part 2.—In order that new generic and subgeneric names appearing in Volume I may all bear the same date, a synoptic key to those to appear in Part 2 is given here.

a. Penis without a sheath, the penial retractor and the vas deferens terminal; no flagellum

b. A well developed epiphallus present; aperture trilobed

Trilobopsis new genus, type Helix loricata Gld.