tions contain many new name-combinations, but little else of present value. Why give storage to useless lumber? I regret that references to most local faunal lists have also been omitted. To quote them for the widely spread species would often require several pages, and the information is in large part covered by the Distribution paragraph under each species.
In dealing with little-known and recently described species my aim has been to give references to all published information.
Misprints of names are not noticed except when likely to cause confusion.
When a species has been named for a locality or a collector mentioned in the Distribution paragraph, it has been thought superfluous to give derivation of the specific name separately. In some other cases the derivation was omitted inadvertently, or in a few it is not known to me.
Dialectics relating to generic nomenclature have been excluded when reference could be made to published discussions. In a few cases some consideration of nomenclatorial matters has appeared necessary.
Species and Subspecies.—1. Many of the species of the families treated in this volume are so well defined that their status has never been seriously questioned; but in all large genera there are races which some malacologists will consider species, others subspecies, according to whether they incline towards splitting or towards lumping. The only practicable criterion of species, in most snails and other organisms, is a discontinuity with allied races in some detail or details of structure; the questions of sterility of hybrids, etc., usually not being available. In species of reasonably conservative genera, such as Mesodon, the absence of actual intergradation of characters may safely be considered specific, even when the divergence of the forms in question, though constant, is small.
2. Subspecies are theoretically races showing some intergradation with neighboring forms in a small proportion of the individuals, but characterized by having a definably different distribution, geographic or ecologic. This distribution may be contiguous to that of conspecific races, or it may be isolated by geologic, climatic or other conditions, as when races are confined to caleareous soils, to humid places in an arid region, or are insular. In practice, the demonstration of complete intergradation with other races is not insisted on. Most subspecies are recognizably differentiated populations which are not considered sufficiently distinct to be called species. They are merely incipient species, in which the discontinuity is incomplete, or is not strongly pronounced. In some cases, that of Triodopsis tridentata juxtidens for example, the geographic or ecologic segregation which a subspecies should have is not evident. Such subspecies are much in need of further study.
Subspecies may be rather distinct over large areas, like the trans-Mississippian Triodopsis albolabris alleni, or they may be indefinitely
