THE NAUTILUS. 119 



that work, making it, if possible, still more valuable to stu- 

 dents of American mollusks. 



To recapitulate : generic names are those by which animals are 

 catalogued, therefore should not be changed without overwhel- 

 ming evidence in favor of the change. This value of the genera 

 in cataloguing has not been sufficiently emphasized. 



A PROVISIONAL KFY TO THE SUBGENERA AND SPECIES 

 OF NORTH AMERICAN LYMNAEIDS. 



BY HAROLD S. COLTON. 



The following is a preliminary key to some of the best known 

 species in the genus Lymnaea. It is based on a careful study of 

 the wealth of material in Baker's Lymnaeidae of North and 

 Middle America. To this the writer has added a little that has 

 come under his observation during the past eight years that he 

 has been working with this group. 



It must be noted that L. auricidaria appears in two places in 

 the key. This is because the radula of the Philadelphia form 

 is different from the radula of the Chicago form. 



It must be remembered also, as stated in the preceding article 

 that the radula and character of the reproductive organs are the 

 important characters in classification as the shell is such a gen- 

 eralized form. This being the case it will be of value in locat- 

 ing but 33 species and varieties as the other 73 have been de- 

 scribed by shell characters alone. When they have been stud- 

 ied the key no doubt will have to be much modified. 



The writer found it impossible to sub-divide the groups in his 

 key. The species within a group are distinguished by shell 

 characters alone, and although one familiar with species can dis- 

 tinguish them without much difficulty, it is almost impossible 

 to state the differences in a few words. 



This key is intended as a supplement to that of Baker p. 125. 

 It makes no pretense of completeness. The writer, therefore, 

 would be glad to hear of any practical difficulties arising in its 

 applicaton. 



In the following key the ratio between the aperture length 

 and the shell length is referred to as the " ratio." 

 GENUS LYMNAEA :- 



Fresh-water pulmonate mollusks. Shell : normally dextral, 

 rarely sinistral, ovately oblong to elongate. Animal: with a short, 

 wide, rounded foot. Tentacles : flattened. Central tooth of the 

 radula unicuspid ; laterals bi-or tri-cuspid. Male and female 

 genital openings separate and on the right side. Mantle margins 

 contained within the shell. 



