384 



lie. The ciliated tentacula, like those of Pesicularia, are eight in 

 number, and do not possess the motionless hair-like processes which 

 project from the back of each in Bowerbankia. 



Although too much importance must not be attached to the actual 

 number of tentacula surrounding the oral aperture, the tendency to 

 multiply those organs must not be altogether forgotten. Thus, while 

 there are but eight in Vesicularia, Bowerbankia densa and Bower- 

 bankia repens possess respectively ten and twelve. 



Both Bowerbankia and Vesicularia agree in the uniserial and uni- 

 lateral distribution of the polypes, but in the present instance the 

 cells are arranged in linear and bilateral clusters. 



February 26, 1857. 

 The LORD WROTTESLEY, President, in the Chair. 



The following communications were read : 



I. " Observations on the Natural Affinities and Classification of 

 Gasteropoda." By JOHN DENIS MACDONALD, Assistant 

 Surgeon R.N. Communicated by Captain DENHAM, R.N., 

 F.R.S. Received January 33, 1857. 



(Abstract.) 



During his sojourn among the Feejee Islands, the author devoted 

 much time to the anatomical investigation of recent Gasteropoda, with 

 the view of discovering such indications of affinity in the details of 

 structure as might serve as a basis for a natural arrangement of the 

 order ; and the present paper is designed to give a statement of some 

 of the results of his researches, in order that the affinities of struc- 

 ture developed may be fairly examined and taken for what they are 

 worth as principles of classification. 



After pointing out objections to the foundation of primary divisions 

 among the Gasteropoda on characters derived from the shell or from 

 modifications of the respiratory organs, the author observes in respect 

 of the value of sexual characters, that when the distinguishing fea- 

 tures of a class are once satisfactorily determined, and this contains 



