38 The Molluscan Family Plaiwrbidae 



two from Comanchean, five from Upper Cretaceous, eight from Eocene, 

 six from Oligocene, five from Miocene, thirty from Pliocene, and five 

 simply described as 'Tertiary.' 



It is obviously very difficult to place fossil mollusks in appropriate 

 genera in a classification based on anatomical characteristics. Nearly all 

 of the fossil planorbids of America and Europe have been described as 

 'Planorhis.' With the greatest of care it is possible to place some of these 

 species in their correct genera as understood at the present time. Some 

 forms, however, can only be tentatively i^laced, basing the decision on the 

 similarity of the fossil species to some known living species of which the 

 anatomy is known. Even among modern species tliis method is fraught 

 with danger because of similar resemblances of the shells of several genera, 

 as in Planorbula and Tropicorbis, where some species of the latter genus 

 have internal lamellae in the aperture similar to species of the genus 

 Planorbula. The two genera are far ai^art anatomically. 



Among American fossils the following genera are known to be repre- 

 sented by several species and races: 



Tropicorbis, 1 species; Paraplanorbis, 1 species; Platytapkius, 2 species; Gyraulus, 

 6 species; Helisonia, 8 species; Carinifex, 2 species; Vorticijex, 8 species; Perrinilla, 

 2 species; Parapholyx, 2 species; Pompholopsis, 1 species; Menetus, 5 species; 

 Planorbifex, 1 sjjecies. 



Many of the species now known as Planorbis may later be placed in more 

 appropriate genera. It is proposed in Part II of this work to review criti- 

 cally all of the American fossil Planorbidae, including several from the 

 West Indies and Central America, and to endeavor to place each species in 

 its correct genus.* 



The geographical distribution of the extinct species of the family Pla- 

 norbidae is not very different from that of the recent species of the family. 

 Fossil species of the family have been recorded from Europe, Asia, Africa, 

 Australia, some of the islands of the Pacific Ocean, North and South 

 America, and the islands of the West Indies. 



B. GEOGRAPHICAL DISTRIBUTION 



Like the Lymnaeidae, the Planorbidae are world-wide in their geographic 

 distribution. There is scarcely a known region in which some of the pla- 

 norbid snails do not live. The distribution by genera, however, is more or 

 less limited. Typical Planorbis and Planorbarius are mostly confined to 

 Europe and western Asia, wdth possibly some forms of circum-Mediter- 

 ranean distribution. Helisoma is wholly American but is known from both 

 North and South America and from some of the West Indies. A fossil 

 species has been reported from northeast Siberia (Lindholm, 1933). This 

 genus is numerous in North America, principally the northern part of the 

 United States and Canada. Planorbula is found only in eastern North 

 America from Alabama northward. Carinifex and Parapholyx are west 

 American in distribution. Menetus is American and does not extend 

 south of the United States. A small group of Helisoma (Seminolina) oc- 

 curs only in the peninsula of Florida. 



*As explained elsewhere, Mr. Baker's death prevented him from completing Part II 

 of this monograph. 



