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72 American Seashells
series of populations on the west coast of Florida which seem to be attempt-
ing a "break-away" from the typical form, and to this geographical race the
name Melojigena corojia perspectiva has been given. Perhaps in another
million years, through fortuitous isolation (geographical or reproductive)
and selection, it will merit recognition as a full species. Elsewhere through-
out the range of corona, we find minor groups of variants, some that are
individuals stunted by ecological conditions, others that are minor genetic
variations which seem to crop up at random in all parts of Florida. These
ecotypes, aberrations and varieties, although actors in the evolution game, do
not warrant subspecific names.
There have been many attempts to define a species. A very excellent
summary of the various definitions has been published in Ernst Mayr's inter-
esting book entitled Systematics and the Origin of Species (Columbia Uni-
versity Press, 1942). Mayr defines species as groups of actually or potentially
interbreeding natural populations, which are reproductively isolated from
other such groups by geographical, physiological or ecological barriers. Un-
fortunately, this biological concept of species cannot as yet be used exten-
sively in the field of mollusks, for malacology is largely in the purely descrip-
tive and cataloging stages, and the majority of species being described today
are still based on the old-fashioned morphological species concept.
While the species is considered by some people as an objective entity in
nature, nearly everyone agrees that a genus is merely a convenient and
arbitrary grouping of closely related species. This is also true of many higher
categories such as the subfamily and family which are merely convenient
groupings of closely related genera. However artificial, the system is ex-
tremely useful, for it permits us to arrange the species in our collections and
our scientific reports in a logical, evolutionary and biological sequence.
IDENTIFICATION FEATURES
These are the many morphological features exhibited in mollusks which
are used for identifying species and in understanding the evolutionary rela-
tionships existing between members of the higher categories, such as genera,
families or orders. It must be realized that in some groups of shells certain
types of characters, such as number of spines, shape of aperture or color
markings are used to distinguish species, while in other groups these will
prove useless and reliance may have to be put on the number of folds in the
columella, the number of teeth in the aperture or the sculpturing on the
operculum. These key features are pointed out in their appropriate places
throughout this book.
The \'erbal tools which are used in the study of mollusks are especially
designed to assure a method as accurate as possible for telling apart the