NOTES ON PLANOKBIS TRUNCATUS MILES.* 



Frank Collins Baker. 



On page 291 of the ** Mollusca of the Chicago Area" the 

 writer stated his opinion that Planorbis truncatus and Plan- 

 orbis trivolvis were closely related, the latter being thought 

 to be the ancestor of the former. A detailed study of the 

 two species has brought to light some interesting facts which 

 seem of enough importance for publication. 



The principal characteristics of truncatus are its very flat 

 spire, angulated whorls and strong spiral sculpture, the latter 

 being confined to the base and periphery, but not appearing on 

 the upper surface of the whorls. These lines are strongest 

 on the periphery and become fainter as they approach the 

 wide umbilicus. The flat spire is bordered by a strongly 

 raised keel, and the lines of growth are strong, elevated and 

 more or less curved. The color of most specimens examined 

 is light horn, similar to some of the Polygyras. 



While examining several thousand Planorbis trivolvis under 

 the microscope, it was discovered that all of the young shells, 

 up to and including the second whorl, were marked with spiral 

 lines in precisely the same manner as truncatus, so that the 

 presence of these lines is not a sufficient character by which to 

 separate the young of the two species, as both have flat 

 spires and spiral lines. There are several characteristics, 

 however, which will separate even the young of these two 

 species. The first is the conspicuous size of the whorls, 

 those of truncatus being about twice as large as those of trivolvis 

 up to the third whorl, when the latter increases in a much 

 larger ratio. Truncatus has 4 whorls when full grown, while 

 trivolvis has 4i to 5 (although in all the books it is de- 

 scribed as with 4 whorls). 



* Presented by title to The Academy of Science of St. Louis, January 18, 

 1904. 



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