10 PHYSAD.E. 



this constitutes a natural family, the only dif- 

 ference of opinion arising upon the order in 

 which they should be placed. Mr. Swainson 

 places Planorbis at the head, because the shell 

 represents that of Nautilus; a consideration 

 which would have prevented me from giving it 

 this position; because an animal cannot represent 

 a different one, however high, and be the typical 

 representative of its own family* The principal 

 difference of opinion, however, has been upon 

 the question whether Ancylus does or does not 

 belong to the family. In one work, it is con- 

 sidered a Patella; in another, a Crepidula; 

 whilst one party asserts that it breathes air, 

 another, that water is its only respiratory me- 

 dium. Amidst this contrariety of opinion, the 

 principal modern authors have placed the genus 

 in the family now under consideration, but more, 

 I suspect, on account of the great similarity of 

 the soft parts, than from any certainty as to 



* Mr. Swainson places Corvus at the head of the in- 

 sessorial birds; and if the carnivorous genus nearest allied 

 to it, is to be considered typical, Buteo will stand above 

 Falco, among the falconine birds ! The genus Perca stands 

 as the representative of its class, although there are many 

 genera which are more nearly allied to the reptilia. 



