ANATOMY OF PALUDBSTRINA 



173 



noted that in this form and Bythinella the ciHa are not 

 distributed all over the lamellae as in Vitrei la but are 

 concentrated at the apex. Each plate shows longi- 

 tudinal folds towards its basal part, becoming flatter where 

 they join up with the efferent vessel. In Bythinella, 

 V i t r e 1 1 a , and Paludestrina jenkinsi the lamellae 

 are flat and unfolded. This folding seems rather difficult 

 to explain. Were it not for the fact that similar folding 

 occurs in P. ulvae (Henking) I would be inclined to think 



Text-fig, 6. 



crv 



Gills in section, av, afferent vessel ; ev, efferent vessel ; pv, pul- 

 monary vein. 



that it might be due to shrinkage arising from excessive 

 contraction of the transverse muscles in each filament. But 

 in addition to the occurrence of similar folds in P . ulvae 

 there is the fact that, if it were due to shrinkage, one would 

 expect such contraction to take effect over all the lamella, 

 which is not the case. If this ultimately proves to be an 

 invariable character of these two species of Paludestrina 

 it may very well be correlated with their brackish-water and 

 marine habitat. It should also be pointed out that unless 

 the text-figures G and H in Bregenzer's paper are diagram- 

 matic the afferent vessels and blood-spaces are much smaller 

 in P. ventrosa than in Bythinella. 



N 2 



