248 HUGH WATSON. 



If the views whicli I have brought forward are correct, all 

 the carnivorous genera of slugs, with the exception of those 

 belonging to the T r i g o n o c h 1 a m i n oe and to the Rathouisi- 

 idte, have been derived independently fi'ora different snails 

 by the pai-allel degeneration of the shell. This will be made 

 clear from the following table : 



American Oleaoinidte — > Strebelia. 



European Oleacinidas •-> Testacella. 



Southern Hhytididee — > Schizoglossa . 



Northern Rhytidida3 - — — -^ Apera. 



European Zonitinfe — > Daudebardia. 



Atlantic Vitrininfe > Plutonia. 



Transcaucasian Parmacellinas > Trigonochlamina?. 



Oriental Ditremata > Rathouisiidse. 



But even if we admit that Apera has probably been evolved 

 from the Rhytidid^eand Testacella from the Oleacinid as, 

 the question of the affinities of these genera is not yet settled ; 

 for Pilsbry^ states that the Oleacinidee and Rhytididas 

 are closely related to each other, and if this be the case 

 Apera and Testacella might still have been deriv^ed from 

 a common predaceous ancestor, instead of their carnivorous 

 characters having been acquired independently. 



Now it is true that the Oleacinidfe and the Rhytididie 

 have many characters in common, but if we except those 

 which would be likely to be caused by their carnivorous habits 

 the remaining features are chiefly such as are possessed by 

 nearly all the more primitive sigmurethrous Stylommato- 

 phora. And there are at least two important differences 

 between the families — their distribution and their shells. The 

 difference in distribution, however, is less important than it 

 appears, for, on the one hand, it is quite conceivable that the 

 Oleacinidee may have once inhabited tropical Africa, and, 

 on the other hand, we have already seen that the Rhytididge 

 may occur in tropical America. But the difference in the shells 

 cannot be so easily explained away. In the Oleacinidte the 

 ' Op. cit., vol. xix, p. xiii. 



