184 GUY C. ROBSON 



Paludestrina agrees with Pterocera, Tiphobia, 

 Lithoglyphus, and a few others in possessing a crystalHne 

 style. We may assume, however, that this is without phylo- 

 genetic significance within the group. 



Another interesting tendency which may not be of phylo- 

 genetic importance is the possession of a single otolith in the 

 Paludestrinidae. It shares this character with Littorina, 

 T r 11 n c a t e 1 1 a , some M e 1 a n i a s , and N a t i c a . On the 

 other hand, Paludina, Ampullaria, Valvata, Cyclo- 

 phorus, and others have multiple otoconia. A blood-gland 

 is absent from the kidney of Paludina, Valvata, Ceri- 

 thium, &c., and is found in Littorina and in the Paludes- 

 trinidae. 



Finally, while possessing a simple osphradium, Paludes- 

 trina exhibits a definite osphradial ganglion — a stage appar- 

 ently more advanced than such forms as Littorina and 

 B i t h y n i a, in which (Bernard, 2) no osphradial ganglion is found. 



5. Summary. 



1. Paludestrina ventrosa possesses the general 

 Taenioglossate organization. 



2. It represents a genus of Paludestrinidae equivalent to 

 Bythinella and Vitrella. 



3. It is peculiar within the family as possessing: 



i. Folded gills ; 

 ii. A slit connecting the style sac through nearly all 



its length with the intestine ; 

 iii. A typhlosole ; 



iv. A non-ciliated roof to the median part of the 

 pharynx. 

 It represents an intermediate stage in the acquirement of 

 the fresh-water mode of life, being essentially a brackish- water 

 form with a fairly well-marked euryhahne tendency. 



4. Several structures not fully described by previous authors 

 are discussed in this paper (e. g. the accessory female and circu- 

 latory organs), and it is not certain in what form these struc- 

 tures occur in other Taenioglossa. 



