VI. 



A NEW CLASSIFICATION OF THE 

 FAMILY PLANORBIDAE 



GENERAL DISCUSSION 



DURIXG THE PAST FIFTY YEARS, the classification of the land 

 shells (Stylommatophora) , under the able leadership of Dr. Henry 

 A. Pilsbry, of the Academy of Natural Sciences of Philadelphia, 

 has been brought to a high degree of accuracy and completeness. This has 

 been accomplished by the study of anatomical details of the genera and 

 species, bringing together groups which are allied by features of the natural 

 organization of the animal. While the shell has helped in many cases it 

 has been the internal features, chiefiy the genitalia, that have given criteria 

 for generic and higher distinctions. The results obtained by these studies 

 have been accepted by nearly all malacologists and conchologists. 



No such comparable work has been done on the fresh-water pulmonates 

 (Basommatophora), and the few attempts to apply anatomical criteria 

 for generic distinctions, as used in the Pulmonata, have met with more or 

 less criticism. It is obvious, however, that only by the careful study of the 

 internal organs of the animal can a natural classification be secured. This 

 was attempted by the writer for the Lymnaeidae in 1911 and some grati- 

 f^'ing results were obtained. The family Planorbidae offers better charac- 

 teristics for purposes of classification than does the Lymnaeidae, there 

 being a greater diversity of structural features among the different genera. 



For the data in the present work a large number of species of the family 

 Planorbidae, from different parts of the world, were dissected. These repre- 

 sent a majority of the generic groups so far published, in addition to 

 several new genera. As in the study of the land mollusks, the genitalia have 

 been of the greatest assistance. Few new groups were necessary because 

 those already published, of which there were many, were adequate to 

 represent the different anatomical features involved. 



In the new classification here presented, great stress is laid on the 

 differences in the male genitalia, the size of the preputium and vergic sac; 

 the characteristics of the prostate diverticula and the prostate duct; the 

 presence or absence of a stylet on the verge; flagella present or absent on 

 the vergic sac; a penial gland in the preputium and whether this has an 

 internal or an external duct or none at all; the opening of the sperm 

 canal in the verge, whether at the end or on the side; the number of 

 retractor muscles of the penial complex; and the nature of the ovotestis, 

 whether the diverticula are in a double series or are multiple. 



Other anatomical features also aid in classification, as the shape of 

 the kidney, whether it is smooth or has a ridge; the condition of the jaws, 

 whether simple or fragmented; the form of the radula teeth; the shape of 

 the stomach and the turns of the intestine from stomach to anus. The 

 pseudobranch also offers some criteria although this organ is fairly uniform 

 in the family. It does effectively separate the Planorbidae from the Bu- 

 linidae, the pseudobranchs of the two groups being markedly different. 

 For generic purposes, the shell offers less assistance than the organs of 

 the animal, although the shell sometimes presents characteristics of generic 

 importance. 



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