THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 237 



improbable that forms more like Rhytida intequalis may 

 also have ooce extended into that region, and given inse, by 

 the degeneration of the shell, to Apera, just as in New- 

 Zealand another branch of the family has probably given rise 

 to Schizoglossa. 



It is not unlikely that the Rhytidid^ originated in early 

 Mesozoic times, or perhaps even before the end of the Pala3o- 

 zoic era, in Grondvvanaland — that great Southern continent 

 which is supposed to have extended from Australia and New 

 Zealand across the Indian Ocean, through Africa, and even 

 as far as South America.^ A little later the more highly 

 specialised Streptaxidae may have arisen in the same 

 region. These did not reach Australia, perhaps because it 

 was alread}^ cut off by the sea, but in other regions we may 

 suppose that they would enter into competition with the 

 Rhytidid^. In South Africa, where only the small pupi- 

 form Streptaxidae occur, the Rhytidid^ were able to 

 withstand their competition by becoming more specialised 

 themselves, either by the degeneration of the shell (Apera) , 

 or merely by an increase in the size of their teeth and a slight 

 modification of their reproductive organs (Natalina). But 

 further north, where we find the heliciform Streptaxidae, 

 with their oblique columella, the Rhytididas were almost 

 entirely exterminated, only leaving Natalina morrumba- 

 lensis [M&P.), N. permembranacea Preston, and pos- 

 sibly the species of Tayloria,^ as relics of their former dis- 

 tribution. Whether the Rhytididfe ever reached South 

 America is at present unknown. They may have done so, 

 and have then been exterminated by the heliciform Strep- 

 taxidae, which are not uncommon in South America. I 



' Hedley has suggested that the Rhytididse are of Antarctic origin 

 C Proc. Linn. Soc. N.S.W.,' 1899, vol. xxiv, p. 398), but, while admitting 

 the possibihty of this theory, I agree with Pilsbry in being unable to 

 find any evidence in its favour (' Rep. Princeton Univ. Exped. Pata- 

 gonia. 1896-1899,' vol. iii, Zool.. 1911. p. 631). 



•■= Thiele, J., 'Deutsch. Zentral-Afrika Exped. 1907-8,' vol. iii, 1912, 

 p. 187. 



