The taxonomic characters in the Planorbidae, their theoretical background and the general basis of their application have been discussed. The various characters have been critically reviewed. In practice the male copulatory organ and the radula have proved to be the most important structures for phylogenetic studies in the family. The prostate comes next in importance. Occasionally the pallial ridges, the jaw, the general shape of the shell, the central nervous system and the lateral appendages are important as characters for comparative studies. More rarely still other features are useful.
Representatives of almost all the planorbid genera have been examined morphologically and their phylogenetically important characters have been accounted for. The material examined includes the type species of twenty genera.
On a comparative morphological basis the family Planorbidae has to be divided up into three subfamilies, the Plesiophysinae, the Bulininae and the Planorbinae. The Plesiophysinae are aberrant and well separated from the other subfamilies. The Bulininae are in the first place characterized by the presence of an ultra-penis. The evolution of the ultra-penis structure from the ordinary basommatophoran penis type is explained. Finally, the subfamily Planorbinae includes the greater majority of the planorbid genera.
The genera of the subfamily Planorbinae are shown to form nine groups. For various reasons the following characters can be regarded as more or less primitive in Planorbinae: a spired, sinistral shell, a moderately concentrated central nervous system, long marginal teeth with both posterior and lateral cusps, the presence of pallial ridges, a terminal pore on the penis, the absence of a flagellum or other appendages on the copulatory organ and a simple prostate. Amerianna on the one hand and Physastra with Miratesta on the other seem to be most closely related to the primitive planorbine form though they do not have all the primitive features. The remaining groups can be arranged in two main branches characterized by two different types of marginal teeth in the radula. The branch characterized by short marginals includes the genera which can be grouped under the Planorbis tribe and the Segmentina tribe as well as the genus Acrorbis. The branch characterized by long marginals includes the genera which can be grouped under the Biomphalaria and the Helisoma tribes. The genera Camptoceras, Drepanotrema and Fossulorbis may perhaps also belong to this branch.
In the Segmentina tribe and in the Helisoma tribe an accessory praeputial organ and an accessory duct between this organ and the upper end of the praeputial lumen or the penis sheath has evolved. These structures have almost certainly evolved independently in the two tribes. The probable origin of the structures has been reconstructed.
The most likely relationships within the family Planorbidae are presented diagrammatically in the figs. 181, 200 and 209.
The phylogenetically based classification present in this paper is critically compared with F. C. Baker's classification of the Planorbidae. Some remarks on the validity of various genera are also given.
A synopsis of the broad distribution of the conventional planorbid genera is given in fig. 210. It shows a fairly good correlation between relationship and distribution of the genera.