THE NATURAL HISTORY SURVEY. 267 



long, reaching below the lower margin of the base of attach- 

 ment, the side cusp smaller; the eight to sixteen laterals are 

 tricuspid, the inner cusp very small; these may be called inter- 

 mediate marginals; marginals at first (seventeen to twenty) 

 modified laterals, with a long, bifid inner cusp and two very 

 short outer cusps; balance ot marginals long and narrow, ser- 

 rated, generally three short cusps at the distal end and two 

 short cusps at the outer side; these latter disappear toward the 

 outer part of the membrane (28-45); a ll have cutting points, 

 especially well developed on the laterals and first marginals 

 (Fig. 85). 



Genitalia: Not examined. 



Distribution: New England to Iowa, Canada, Manitoba 

 and California, south to Virginia, Kentucky and New Mexico. 



Fig. 85. 



Radula of LIMN.EA DESIDIOSA Say. (Original.) c, central tooth; 

 1, first lateral; 8, lateral tooth; 17-20, modified marginals; 21-45, various 

 types of marginals. 



Geological distribution: Pleistocene; Loess. 



Habitat: In small bodies of water, clinging to submerged 

 stones and sticks. It occasionally inhabits the larger 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 variations 

 and numerous names have been given to these forms.* In the 

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



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

 tions, that several other species will have to become synonyms o\desidiosa; L. cfaussaSay 

 and L.fusiformis Lea may be considered doubtful species. 



