182 GUY C. ROBSON 



fera. The rest of the contents were usually too much digested 

 to enable their nature to be made out. No remains of plant 

 fibre, &c., was ever found. I am inclined to think that it 

 browses upon the microfauna and microflora of the plants upon 

 which it lives, and that it does not actually chew the leaves of 

 the latter. 



4. Affinities. 



(a) I cannot agree with Bregenzer's verdict upon the imme- 

 diate relationships of Taludestrina (6, p. 276). According 

 to her, the latter genus is separated into a group distinct 

 from Bythinella and Vitrella upon the following 

 characters : 



(1) Fusion of the cerebral and pleural ganglia. 



(2) The possession of two pairs of salivary glands. 



(3) Eeduction of the ' Knorpelspange ' of the lingual cartilages. 



(4) Brackish-water habitat. 



Of these characters the first is open to question. In P . v e n - 

 t r o s a there is no more fusion of the ganglia in question than 

 in Bythinella, while in P. ulvae we have seen (p. 168) 

 that Henking's statements are open to question. In the second 

 place, only one pair of salivary glands is found in P . v e n t r o s a. 

 As to the third character it is scarcely worth anything as the 

 ' Knorpelspange ' is absent in Vitrella! Lastly, P a 1 u d e s - 

 trina is not restricted to brackish water, at least as far as 

 England is concerned. As the result of a scrutiny of the 

 characters available for taxonomic purposes we might with 

 equal justification select the simple penis and longer super- 

 intestinal part of the visceral commissure in order to unite 

 Paludestrina with Vitrella as against Bythinella, 

 or the crystalline style and certain features of the radula to 

 unite Paludestrina and Bythinella against Vitrella. 

 In any case I venture to think that an animal's taxonomic 

 position cannot be summarily decided in this fashion. Until 

 we have objective evidence as to the taxonomic value of the 

 various characters such groupings as those discussed above 

 are of little value. On the whole we can safely consider these 



