IV. 



DISTRIBUTION IN TIME 

 AND SPACE 



A. GEOLOGICAL HISTORY AND DISTRIBUTION 



THE ANCESTRY of the Planorbidae, as of other members of the 

 puhnonate molkisks, is shrouded in mystery. It is said that the 

 ancestors of the Planorbidae, as of other groups of the Basom- 

 matophora, were originally aquatic, then migrated to the land and finally 

 again returned to aquatic habitats. Perhaps this was accomplished through 

 such groups as the Auriculidae {Carychium for example) which have the 

 Basommatophorous organization but inhabit the land. 



In America the family apparently first appears in the Morrison forma- 

 tion which is considered of Jurassic or Early Cretaceous age, probably 

 Jurassic. In Europe, a species, Planorbis mendipensis IVIoore, is known 

 from the Liassic of England. The family is not uncommon in the Bear 

 River formation, believed to be at the top of the Lower Cretaceous 

 (Comanchean), or at the base of the Upper Cretaceous, and also in the 

 several divisions known to belong to the Upper Cretaceous, as Judith 

 River, Belly River, and others. During Tertiary time the family was well 

 represented. Eocene, ]\Iiocene, Oligocene, and Pliocene formations have 

 yielded many characteristic species. The Pleistocene planorbid fauna is 

 practically the same as the recent fauna. 



As the few specimens of planorbid shells from Jurassic or Lower Cre- 

 taceous time are not notably different in general characteristics from those 

 of later geological periods or from the modern fauna, it is obvious that the 

 family in some form must have existed in earlier strata, Triassic and 

 Carboniferous (Pennsylvanian and Mississippian) . A Physa (prisca Wol- 

 cott) and other fresh-water groups are reported from the Mississippian of 

 Nevada and fresh-water and land mollusks are known from various locali- 

 ties from strata of Pennsylvanian age. Planorbids, however, have not yet 

 been reported from these older geological periods. 



In Europe this family is fairly common in rocks of IMesozoic and 

 Cenozoic age, but earlier records are very few in number. A Triassic pla- 

 norbid has already been mentioned. In the Upper Oolite another species 

 appears, Planorbis fisheri Forbes. In the Eocene a number of species of 

 this family occur, and in the Oligocene more than eleven species are 

 known. The family is not uncommon in later periods. On the continent a 

 large number of species and races of this family have been recorded from 

 nearly all of the later geological periods. Wenz (1923) has listed all of 

 these but many of the species have obviously been placed in the wrong 

 genera, especially some of the large forms which are listed under the genus 

 Planorbina , for it does not occur in Euroi^e. It is cjuite probable that these 

 large species are members of the genus Planorbarius and are related to 

 the large Planorbarius corneus so common in the recent fauna. 



In America (see Henderson, 1935) sixty-three species of fossil planor- 

 bids have been described. Of these two are of Jurassic or Comanchean age, 



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