PHYSADiE. 9 



must, in general, be considered as assumed, 

 rather than as demonstrated. As linear ten- 

 tacles appear to constitute the normal form 

 among the mollusca, their triangular form in 

 Limnea, would seem to point out this genus as 

 presenting the most important distinctive cha- 

 racter; but on similar grounds we might place 

 the Helicinse above this family, on account of 

 the situation of their eyes, and the possession 

 of an additional pair of tentacles; whereas, they 

 are much less highly organized than the Lim- 

 neans;* and we might place Ancylus at the 

 beginning of the latter family, because it is the 

 uniting link between the hydrobranchiata and 

 pneumobranchiata. 



Malacologists are universally agreed, that 

 with the genera Physa, Limnea, and Planorbis, 



* In a cancelled portion of my manuscript, written under 

 the impression that Limnea should stand at the head of the 

 family, I remarked that "I at one time considered the 

 genus Physa to stand higher than Limnea, judging from 

 their habits, and considering the mantle as indicating a 

 more complex organization; but these views were counter- 

 balanced by the belief that sinistral animals do not occupy 

 a more elevated station than their congeners, because, from 

 the rarity of reversed genera, they may be viewed some- 

 what in the light of monstrosities." 



i 2 



