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 



