PALUDINA DECISA. 7 



rosa. It must be confessed that the two are 

 nearly allied, but I think it would be immature 

 to unite them until a complete series of the 

 shells of both could be shown, which would 

 indicate the passage of the adult of one into 

 that of the other species. P. decisa has a 

 thinner shell and narrower aperture; nor are 

 the lines of growth so suddenly recurvent as in 

 P. ponderosa. The young have a near resem- 

 blance, but the larger the decisa grows the less 

 is the resemblance apparent. 



Valenciennes cites Say's description of P. 

 limosa (Journal of the Academy, vol. i. p. 125) 

 for this species; and, leaving size out of the 

 question, descriptions of the two shells would 

 very nearly correspond. M. Valenciennes' spe- 

 cimens are about an inch in length, the truo 

 limosa being no longer than 0.15 of an inch. 



This species has been found heterostrophe 

 in Ohio, the Delaware, and in Vermont; and 

 upon this variety Professor Kirtland has found- 

 ed his species. 



REFERENCE TO PLATE 1. 



Fig. 1 represents an animal from the Susquehanna, 

 where the shells are shorter than usual. Fig. 2 is taken 



