BY H. LEIGHTON KESTEVEN. 623 



I would not be understood to advance this theory as a convic- 

 tion; it is but a speculation of one who has not been able to 

 examine the organ for himself; moreover, the osphradium cannot 

 be regarded as functionless, for, as already mentioned, it is 

 highly innervated. 



LlTTORINID^. 



The family Littoriaidce has, at my hands, suffered only restric- 

 tion. The reasons for removing the various genera therefrom 

 will be found under the families in which I have placed them. 



RiSELLID.E. 



The family Risellidce has been formed for the reception of the 

 two unitypical genera Risella and Risellopsis. 



I proceed at once to describe the anatomy of Risella inelano- 

 stoma, Gmelin. 



External features : — The mollusc extruded from its shell has 

 been figured by Quoy and Gaimard (18), as also the operculum. 

 The ground colour of the body is white, reticulated with black 

 markings, which in places become confluent. Foot white, tinged 

 with yellow. Operculum paucispiral, corneous, oval. The snout 

 is not as long as it is represented in the figure referred to. 

 Tentacles of medium length, carrying the eyes on prominences 

 at their bases. When compared with Littorina or Lotorium, it 

 is seen that Risella is twisted half round in its shell. In the 

 former genera the dorsal aspect of the mollusc is that opposite 

 to the axial, whilst in the latter, and most probably all genera 

 possessing trochiform shells, the dorsal aspect is that which is 

 uppermost when the shell is standing on its base. To make this 

 more plain, when Littorina is set perpendicularly, with the spire 

 up, the dorsal face of the mollusc is on the outside of the coils; 

 with the shell of Risella in the same position the mollusc is lying 

 with its dorsal face up. This twisting of the mollusc in the shell 

 has resulted in the descending of the columellar muscle on to the 

 base of the shell. In the following description I have spoken of 



