292 PROCEEDINGS OP THE MALACOLOGICAL SOCIETY. 



be simplest to suppose that the change in the position of the ocular 

 retractor took place first, before Theba branched ofi from the 

 ancestors of Helicella and Ochthephila, and that the greater opacity 

 of the shell found in most species of the latter genera, and the con- 

 sequent reduction in the pigmentation of the mantle, were features 

 which aiose later, owing to the preference of these snails for less shady- 

 situations. Nevertheless, Hesse is probably right in regarding 

 Hygromia and Theha as closely related, for all these genera seem to 

 be nearly allied to one another. Possibly their mutual relations 

 may be somewhat as follows : — • 



Helicella Ochthephila 



Hygromia >^ ^ Theba 



X 



The microscopical sculpture of the shell of Ochthephila turricula 

 and allied species closely resembles that of the type species of 

 Helicigona, but the anatomy of the two genera shows that this 

 resemblance is not due to any close relationship between them. 

 Indeed, it is doubtful whether the reproductive organs of any 

 European genus belonging to the Helicidae are more unlike those of 

 Ochthephila than are these organs in Helicigona. The larger 

 Siphonadeniate Helicids — i.e. Helix, Helicigona, etc. — seem to belong 

 to a slightly different and somewhat more primitive group than the 

 genera dealt with above. In them the kidney appears to be of a 

 slightly different shape, with the mantle-cavity extending further 

 back above it, both the pleural ganglia are still quite separate from 

 the parietal ganglia, the long receptacular duct usually bears a 

 diverticulum, and the dart-sac and raucous glands have not yet 

 become more or less degenerate. But it is possible that the smaller 

 Helicids have not been directly derived from any of the larger genera 

 now existing ; it is even conceivable that they may have been 

 independently evolved from the Euadenia ; for the step is not a 

 great one, and some of the Euadenia, such as the African genus 

 Halolimnohelix, seem to resemble Hygromia and its allies in some 

 respects more closely than the latter genera resemble the larger 

 European Helicids. Probably a comparative study of the central 

 nervous system in these various forms would throw some light on 

 their probable relationships, but unfortunately nothing has yet been 

 published about the nervous system of Halolimnohelix and many 

 other genera. 



It is generally agreed that the MoUuscan fauna of the Madeira 



