LIMNEA FRAGILIS. 23 



as ours are, I cannot tell; but I have specimens 

 which, if found here, would be named elodes 

 without hesitation. The respective soft parts 

 may differ; but until such a difference be point- 

 ed out, I cannot but consider the two identical; 

 nor can I admit doubtful species, or those 

 nearly allied to others, upon a mere (so called) 

 specific character, unless a comparative character 

 be given at the same time. Surely it is not 

 unreasonable to demand that enough be given 

 to convey an idea of proposed species, which, 

 in many cases, are difficult to identify, even 

 from figures. 



Mr. Gray is of opinion that the calcareous 

 rim within the peristome of certain Limneae and 

 Planorbes, is formed when the waters they 

 inhabit are nearly dried up; or in winter, when 

 they are affected by cold preparatory to the 

 secretion of a diaphragm for their protection 

 at these times.* However this may be, I have 

 never observed the rim in any other of our 

 species, and only in variety D. of this, whose 

 appearance will always distinguish it, even 

 when this character is wanting. This variety 

 has not been hitherto observed east of the 

 Allegany Mountains. 



* Turton's Manual, p. 231 and 267. London, 1840. 



