132 Annals of the South African Museum. 



vidae aro3e at the beginning of the Mesozoic era somewhere near 

 the middle of Gondwanaland; and the members of the family spread 

 east and west and south, and underwent upward evolution in the 

 centre. Then that ancient southern continent gradually broke up 

 into four large islands (one of which became subdivided later), and 

 the Acavidae became divided into four subfamilies, one in each 

 region. Lastly, some of these subfamilies began to be exterminated 

 by newer and more highly organized groups of snails. 



But this story of the evolution and distribution of the Acavidae 

 may not be true. Hedley has suggested that the family arose on 

 the Antarctic continent." Now it must be admitted that the climate 

 near the South Pole was once much more suitable for the evolution 

 of snails than it is at present, and it is very probable that the 

 Antarctic continent was at one time united to both Australia and 

 South America. Possibly it may have been united to South Africa 

 also, but it is not necessary to invoke the aid of this highly prob- 

 lematical land-connection before we can accept Hedley's theory. 

 Since the Dorcasiinae are so nearly allied to the Stropliocheilhiae, 

 we might suppose that their ancestors had spread from the /Antarctic 

 to Africa by way of South America, having made use of the old 

 connection between Africa and Brazil. Similarly the Acavinae 

 might have reached their present home from the Australian 

 region. But Hedley's hypothesis does not seem to explain the 

 relative stages of development which have been attained by the 

 different members of the family in so satisfactory a manner as 

 the theory given above; and it certainly involves far greater as- 

 sumptions ; for we have no evidence at all that the Acavidae 

 ever inhabited any part of the region which Iledley assumes was 

 their original home. Moreover, Hedley's theory has been rejected 

 by Pilsbry,t than whom there is no greater authority on the 

 geographical distribution of snails. In writing about the Aca- 

 vidae, Pilsbry goes so far as to state that " the radiation of this 

 scattered group from the Palaeozoic Gondwana continent of 

 Neumayr seems a reasonable, in fact the only tenable, hypothesis." I 

 Far more information, however, is needed about the comparative 

 anatomy of snails before we can attach any great value to theories 

 concerning the origin and evolution of the various families. Pos- 

 sibly further researches may show a more intimate relationship 

 between some of the subfamilies of the Acavidae than at present 



* Proc. Linn. Soc, N.S.W., 1899, xxiv. p. 396. 



t Eep. Princeton Univ. Exped. Patagonia, 1896-99, iii. (1911), p. 631. 



; Rep. Piinctton Univ. Exped. Patagonia, 1896-99, iii. (1911), p. 614. 



