PREFACE 



THE WIDE DISTRIBUTION, large number of species represented, 

 and the economic significance from a medical standpoint, as inter- 

 mcfliate hosts of flukes or trematode worms, constitute the family 

 Planorbidae as one of the most important groups of fresh-water Mollusca. 

 It is the largest and most conspicuous of the families of Basoramatophor- 

 ous pulmonate snails. 



Unlike the terrestrial pulmonates (Stylommatophora Pulmonata) 

 which have been brought to a high state of precise classification from the 

 anatomical studies of Dr. Henry A. Pilsbry and his co-workers, the Basom- 

 matophora are still in a condition of more or less chaos as regards classi- 

 fication, all of the monographs and many of the local studies being based 

 wholly or partly on characteristics of the shell with little regard for rela- 

 tionships which might be gained by a study of the organization of the 

 animal. The condition of this large group as far as classification is con- 

 cerned is little better than that of the land snails seventy-five years ago. 



In 1911 the writer published a monograph of the Lymnaeidae of North 

 and Middle America, basing the classification on characteristics of the 

 genitalia and radula. In that work it was suggested that studies on 

 Planorbis, Physa, and other groups might follow the publication of the 

 Lymnaeidae volume if it should meet with the approval of the conchologi- 

 cal public. That it is still in general use after an interval of more than 

 thirty years is evidence that its reception has been, on the whole, of a 

 favorable nature. 



The present volume on the Planorbidae was begun some twenty-five 

 years ago and has been in active preparation for the past ten years. As 

 the work progressed it became evident that adequate treatment of the 

 subject could not be given in a volume similar to that on the Lymnaeidae 

 and so the work has been divided into two parts, part I on the anatomy 

 and classification of the family, and part II* on descriptions of the species 

 of the family inhabiting North and South America and the West Indies. 



The study of the internal organization of the family Planorbidae has 

 shown that, as in the case of the land snails, the anatomy gives the only 

 true key to a natural classification. The shell alone cannot be relied upon 

 for this purpose because some of the characteristics are often misleading, 

 affording many cases of parallel development, as the presence of lamellae 

 within the aperture, which feature is found in a number of groups which 

 are unrelated anatomically. There are some shell characteristics, however, 

 which are of value in classification when used in conjunction with ana- 

 tomical features. 



For the anatomical studies recorded in this volume, eighty species and 

 races of the family have been examined, represented by more than 725 

 specimens. The anatomy of six additional species has been taken from the 

 literature. The number of species examined has been divided among the 

 four subfamilies as follows: Planorbinae 19; Segmentininae 11; Helisoma- 

 tinae 41 ; Planorbulinae 10. A large majority of the known genera and 

 subgenera are represented and it is confidently believed that additional 

 anatomical study will not greatly alter the classification herein set forth. 



*See title page of Part II (page 213) for statement regarding change in scope of 

 this part. 



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