XVI. A PRELIMINARY CATALOG OF THE NORTH 

 AMERICAN SPH.ERIID.E. 



By Victor Sterki, M.D. 



A monographic revision of the North American Sphaeriidie has long 

 been demanded. Interest in this group of mollusks is steadily increas- 

 ing. Nevertheless the way is not yet clear for preparing such a work 

 in an entirely satisfactory manner. Much is still to be learned con- 

 cerning the limits of some species, their trend and extent of variation, 

 their interrelations, and relations to those of other zoogeographical 

 provinces, and their resultant grouping and distribution. Further- 

 more in some cases we need to ascertain more details of a morpho- 

 logical and anatomical nature, and of the stages of development. 

 More good material from all o\"er the continent is still desirable, 

 especially from those regions where little or no collecting has taken 

 place. More specimens of the fossil species are also needed, especially 

 from the Tertiaries and still older formations, in order that the 

 phylogeny and geological history of the various species may at least 

 to some extent be elucidated. 



The study of this intricate group of mollusks is rendered especially 

 difificult and peculiarly interesting by the great, and in some species 

 apparently almost endless, variation which occurs. It may be in 

 order to state in this connection that injudicious and haphazard 

 "lumping" has done more to bring about difficulties than has been 

 caused by minute discrimination between forms. The careful dif- 

 ferentiation of forms alone enables us to determine their natural 

 interrelations and affinities. Some forms of distinct species may 

 resemble each other closely; while on the other hand forms of one 

 species may be so different from each other, that at first glance they 

 appear to belong to widely different species. 



An essential factor in evolution is the influence of habitat. The 

 Sphaeriidse furnish some very interesting illustrations of variation 

 under the influence of environment. It is therefore very desirable, 

 if not indispensable, to have specimens accompanied by accurate 

 data as to the nature of the localities from which they are derived. 



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