On the anatomy and aflanities of Paludes- 

 trina ventrosa, Montague. 



By 



Guy C. Robson, B.A. 



(Published by permission of the Trustees of the British Museum.) 



1. Introduction. 



The Prosobranch mollusc which is described in this paper 

 is a small insignificant animal found upon plants or bottom 

 debris in the brackish water of creeks and tidal ditches in various 

 parts of England, Wales, and Ireland. It is also found in the 

 upper waters of estuaries. Jeffreys (9) records it from ' the 

 sea coasts of Sweden, France, and Portugal, as well as of 

 Algeria ', though such cases are open to a great deal of doubt. 



It has been selected for study for several reasons. In 

 the first instance there is very urgent need for more informa- 

 tion about the Taenioglossate Prosobranchs to which group the 

 Paludestrinidae are referred. In the second place, though 

 some substantial knowledge is available upon the classification 

 and structure of the Paludestrinidae, the euryhaline habits of 



