TEE CARNIVOROUS SLUGS OF SOUTH AFRICA. 249 



shell is elongate, with a pointed spire and laterally compressed 

 whorls, and the columella is truncated or folded. In the 

 Rhytidid^e, on the other hand, the shell is lieliciforni or 

 depressed, with a very obtuse spii'e and laterally expanded 

 whorls, and the columella is neither truncated nor folded. I 

 fail to see how it is possible to derive the shell of the 

 Oleacinidte directly fi-om that of the Rhytididse or vice 

 versa. If these families are really related, it can only be 

 indirectly, and we must postulate a large number of inter- 

 vening forms, with shells intermediate in shape. 



But it might be asked whether we do not find such a series 

 of intermediate forms in the Streptaxid^e; and as a matter 

 of fact in this family we have every gradation from heliciform 

 shells such as Art em on and Imperturbatia to cylindrical 

 shells like Ennea, and from these to pointed shells with 

 laterally compressed Avhorls such as Streptostele and 

 Obeliscella. May it not be that the Streptaxidge have 

 been derived from the Rhytididse and the Oleacinidte 

 from the Streptaxidfe ? In my opinion the Streptaxid^ 

 may possibly have been derived from the Rhytidid^e, for 

 I have recently dissected a form which proves to be in some 

 ways intermediate between the two families. The great 

 majority of the Strep taxidas, however, have come to differ 

 widely from both the Rhytidida? and the 01 e acini die in 

 their nervous system, their reproductive organs, and even in 

 their radula ; and I think that there can be no doubt at all 

 that the Oleacinidte have not been derived from that 

 family. Therefore, if Pilsbry's view is correct, we must sup- 

 pose that all the forms intermediate between the Rhytidida) 

 and the Oleacinidte have died out completely, which does 

 not seem a probable hypothesis. 



Dr. Simroth has suggested that the Oleacinidte may have 

 been derived from the Achatinidse. Now I regard the 

 striking resemblance between the shell of the American 

 genus Euglandina and the African genus A ch a tin a as 

 almost certainly due to convergence; because Euglandina, 

 with its long labial papillae and closely aggregated nerve- 



