8 



Trans. Acad. Sci. of St. Louis. 



at the outer side ; these latter disappear toward the outer part 

 of the membrane (28-45) : all have cutting points, especially 



well developed on the laterals 

 and first marginals (f. 4). 



Distribution: New England 

 to Iowa, Canada, Manitoba and 

 California, south to Virginia, 

 Kentucky and New Mexico. 



Geological Distribution: 

 Pleistocene; Loess. 



Habitat: In small bodies of 

 water, clinging to submerged 

 stones and sticks. It occasion- 

 ally inhabits the large rivers. 

 Prefers still water, and has 

 been dredged in Lake Superior 

 at a depth of 8 to 13 fathoms. 

 Remarks. This species is 

 subject to some little variation, 

 and numerous names have been 

 given to the forms.* In the 

 main, however, it may be recognized by its long, pointed 

 apex, and elongately-ovate aperture. It approaches L. 

 humilis in some of its forms, but that species always has a 

 shorter, more obtuse spire and a more rounded aperture. 

 The lower part of the aperture in the latter species is not 

 produced as in desidiosa. When in motion the animal is slow 

 and deliberate; the shell is pulled forward by a series of 

 jerks. This is a very common Limnaea and is found in all 

 parts of the area. Found fossil in sand banks on the lake 

 shore north of Graceland avenue. 



$X-VS 



Xt-%1 



Fig. 4. Radula of Limnaea desi- 

 diosa Say. C, central tooth; 1, 

 first lateral ; 1-8, laterals; 17-20, 

 modified marginals; 21-45, vari- 

 ous types of marginals. 



* It is evident from study of present material and the original figures and 

 descriptions, that several other so-called species will have to become 

 synonyms of desidosa: L. obrussa Say and L. fusiformis Lea, may be con- 

 sidered doubtful species. 



