18 The Molluscan Family Plauorbidae 



Altitude does not seriously affect the distribution of the members of the 

 family Planorbidae. In the Rocky Mountains they occur in lakes at alti- 

 tudes of from 9,000 to 10,000 feet. Lake Titicaca, in Peru, which contains 

 many mollusks, including a peculiar group of Planorbidae [Taphius) is 

 at an altitude of 12,846 feet. The Swiss lakes are at comparatively high 

 altitudes as are also some of the lakes of Asia which contain moUusk 

 faunas of some size. 



Quite a literature has accumulated on the general ecology and par- 

 ticular habitat relationship of the Planorbidae and other fresh-water 

 mollusks. The remarks on tlie lymnaeas in the 1911 monograph on that 

 family may be consulted with advantage. A number of special papers and 

 books have been published which bear on this subject. These are listed 

 in the bibliographv and may be consulted under the following names: 

 Adams, 1909; Baker, F. C.^ 1910, 1911, 1916, 1918, 1920, 1922, 1924; 

 Dybowski, 1875; Forel, 1869, 1874; Lindholm, 1909, Tavlor, J. W., 1894- 

 1900. 



VARIATIONS IN DIFFERENT ENVIRONMENTS 



The planorbid snails do not exhibit as much variation due to differences 

 in the environment as do the Lymnaeidae (see Baker, 1911, pp. 29-32). 

 The shell aperture often increases in size in large lake localities where the 

 animal is subject to heavy wave action. This may be observed in Helisoma 

 anceps percarinatum and in Helisoma anceps sayi which are lake dwellers 

 in contrast with typical Helisoma anceps which is usually a river or creek 

 inhabitant. Helisoma canipanulatum davisi is a small pool or swamp 

 dweller while typical Helisoma campanulatum is a lake inhabitant and 

 usually has a larger, more expanded aperture. Some species of this family 

 show preferences for particular types of habitat. Thus Planorbula armigera 

 is usually found in ephemeral pools or small bodies of water, or in pro- 

 tected places in larger ponds and lakes. Planorbula crassilabris may often 

 be found on a lake shore fully exposed to the waves. Promenetus umbili- 

 catellus is an inhabitant of ditches, pools, and swampy places, while Pro- 

 nienetus exacuous is usually found in larger bodies of water. 



Great variation is often seen in specimens of the same species from one 

 habitat. Thus Helisoma duryi seminole, in Lake Eustis and Lake Griffin, 

 Florida, varies from a typical planorboid form to a distinct physoid form 

 by the elongation of the spire. In Lake Titicaca, in Peru, Taphius andecolus 

 exhibits many variations in the form of the aperture, the whorls, and the 

 condition of the umbilical region. In Carinifex the individual variations 

 are numerous among specimens of one species from the same habitat. 

 Among some of the species of the genus Australorbis of South America and 

 the West Indies there is often great variation in the axial height of the shell. 



These examples might be greatly multiplied but enough have been pre- 

 sented to show that variation is the rule among individuals of the family 

 Planorbidae and that this variation may not be due alone to conditions 

 of the environment but rather to the inherent 'law to vary' which is found 

 in all living things. The family Planorbidae does not appear to offer as 

 good a field for the distinguishing of the so-called habitat or ecological 

 variations as does the familv Lvnmaeidae. 



