ANATOMY OF PALUDBSTRINA 183 



genera as referable to the same family ; but I feel that we require 

 another technique for deciding their closer affinities than 

 a mere inspection and assorting of characters. A character 

 such as the absence of the crystalline style in Vitrella 

 would* appear in the first instance to be profoundly important. 

 But we do not know the precise significance of its absence. 



As an alternative to a close study of genetics, evolution, 

 habits, and ecology in relation to structure which alone can give 

 us a sound taxonomic method, the only procedure that could 

 be suggested would be a complete enumeration of characters 

 and a grouping based upon agreement or disagreement in a large 

 number of structures. This method would be crude, but it 

 would be better than an arbitrary selection of a few characters. 

 In the present case I have distinguished a total of twenty-one 

 important characters. The agreement or disagreement of 

 the three genera in question is indicated as follows : 



Paludestrina = Bythinella alone in 5\ 



,, =Vitrella alone in 2| 



,, =Bythinella and Vitrella in 4 21 



,, = neither in 8i 



relationship uncertain in 2) 



(b) Though it would be beyond the scope of this paper to 

 offer a criticism of the present arrangement of the Taenioglossa, 

 we may nevertheless attempt to define the position of the 

 Paludestrinidae with regard to some of the main tendencies 

 of Prosobranch morphology. 



The Paludestrinidae represent a stage in the abbreviation 

 of the nervous system which involves the pleural-intestinal 

 portions, and is seen in its extreme condition in Bithynia 

 and Valvata in which the sub- and supra-intestinal ganglia 

 are either fused or closely approximated to the pleural ganglia. 

 In Melania and Cerithium this condition of close 

 approximation is seen on one side only, the ganglia being 

 separated on the other side. In Paludestrina and 

 Bythinella they are slightly separated on both sides, while 

 in Littorina and Paludina they are widely separated. 



