236 HUriH WATSON. 



to the Rhytididte, ar.d I am inclined to believe thatPilsbry 

 is right. Beutler has examined the histology of the skin of 

 Paryphanta, and has found that the foot is without dermal 

 mucous glands, as it is in Apera. The pedal gland in the 

 Rhytidida3 not only lies freely in the body-cavity, but it 

 is often somewhat contorted, and, as in Apera, it ends 

 in a vesicle containing a broad fold. In Parj-phanta, 

 Natalina, and Rhytida capillacea {Fn\), the visceral 

 ganglia are all separate, as in Testacella, but in Rhytida 

 inaequalis {Pfr.) Fischer has shown that the left parietal 

 ganglion is united with the abdominal ganglion exactly as in 

 Apera. I have already shown that the arrangement of the 

 odontophoral muscles in at least one member of the 

 Rhytididse is very similar to that found in Apera, and, 

 judging from Beutler's account, the structure of the odonto- 

 phoral support is also similar. The reproductive organs of 

 Rhytida and Paryphanta are on the whole very like 

 those of Apera, and in these genera the right tentacular 

 retractor crosses the penis. The members of the Rhytidid^ 

 also resemble Apera in having no secondaiy ureter. Indeed, 

 almost the only differences that I know of between Rhytida 

 and Apera are those which would be likely to be bi'ought 

 about by the degeneration of the shell and its further 

 retrogression to the hind end of the animal, and we have seen 

 that this is a modification which is especially liable to occur 

 in carnivorous forms. 



Another fact in favour of the theory that Apera has 

 been evolved from the Rhytidid^e is to be found in the 

 geographical distribution of that family, for it occurs not only 

 in the Australian region, but also in South Africa itself. It 

 is true that Apera has almost certainly not been evolved 

 from those members of the Rhytidida3 which are now found 

 in South Africa : these have retained their separate visceral 

 ganglia, but have become specialised in another direction, as 

 is shown both by theii- radula and their reproductive system, 

 in which the penis lies to the right of the tentacular retractors. 

 But the presence of these snails in Africa renders it not at all 



