THE GASTROPODA 109 



interpretation, since the anatomy of the animal shows it 

 to be actually sinistral. Other cases are known where 

 the nucleus or protoconch is sinistral and the adult shell 

 dextral, and such shells are termed heterostrophic. 



Many species of Planorbis are found in the same beds, 

 and several are also found living in our streams and 

 ponds, it being a purely freshwater genus. Although 

 differing in details of shape and proportions from the 

 species described above, their general likeness to it 

 is obvious, and in all cases the sinistral character of the 

 young shell can be recognized. 



The classification of the Gastropoda is a very unsatis- 

 factory matter for the palaeontologist. The features 

 which zoologists have found to be the best basis for 

 a natural classification leave no mark upon the shell, and 

 there are not, as there are in lamellibranchs, important 

 shell-characters that can be made a basis for broad 

 divisions. If gastropods were entirely or mainly extinct, 

 we should be driven to make a shell-classification, but it 

 would be quite obviously artificial. As it is, there are 

 few extinct genera which are not obviously related to 

 recent forms, so we have to accept the zoological classi- 

 fication and place these extinct genera with their nearest 

 allies. 



The fundamental division is based upon a difference in 

 the nervous system : in one main sub-class, the Strepto- 

 neura, the twisting of the body which brings the anus 

 round nearly to the head has twisted the nerve-loop which 

 supplies the gills into a figure-of-8 ; in the other, the 

 Euthyneura, it has left it straight. The latter sub-class 



